[package] nlohmann_json/3.7.3: "multiple_headers" option doesn't change package ID
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### Package and Environment Details
  * Package Name/Version: **nlohmann_json/3.7.3**
  * Operating System+version: **Windows 10 1809** (also tested in WSL Ubuntu 18.04.3)
  * Compiler+version: **MSVC 19.16** (gcc 7 in WSL)
  * Conan version: **conan 1.24.0**
  * Python version: **Python 3.7.3** (3.6.9 in WSL)


### Conan profile
```
Configuration for profile default:

[settings]
os=Windows
os_build=Windows
arch=x86_64
arch_build=x86_64
compiler=Visual Studio
compiler.version=15
build_type=Debug
compiler.runtime=MDd
[options]
[build_requires]
[env]
```


### Logs
<details><summary>With multiple_headers set to False</summary>

```
conan info nlohmann_json/3.7.3@_/_ -o nlohmann_json:multiple_headers=False
nlohmann_json/3.7.3
    ID: 5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9
    BuildID: None
    Remote: conan-center=https://conan.bintray.com
    URL: https://github.com/conan-io/conan-center-index
    Homepage: https://github.com/nlohmann/json
    License: MIT
    Description: JSON for Modern C++ parser and generator.
    Topics: conan, jsonformoderncpp, nlohmann_json, json, header-only
    Recipe: Cache
    Binary: Cache
    Binary remote: conan-center
    Creation date: 2019-11-27 02:09:05
```

</details>

<details><summary>With multiple_headers set to True</summary>

```
conan info nlohmann_json/3.7.3@_/_ -o nlohmann_json:multiple_headers=True
nlohmann_json/3.7.3
    ID: 5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9
    BuildID: None
    Remote: conan-center=https://conan.bintray.com
    URL: https://github.com/conan-io/conan-center-index
    Homepage: https://github.com/nlohmann/json
    License: MIT
    Description: JSON for Modern C++ parser and generator.
    Topics: conan, jsonformoderncpp, nlohmann_json, json, header-only
    Recipe: Cache
    Binary: Cache
    Binary remote: conan-center
    Creation date: 2019-11-27 02:09:05
```

</details>

<details><summary>fmt/6.1.2 with header_only True/False for comparison</summary>

```
conan info fmt/6.1.2@_/_ -o fmt:header_only=False
fmt/6.1.2
    ID: 8cf01e2f50fcd6b63525e70584df0326550364e1
    BuildID: None
    Remote: conan-center=https://conan.bintray.com
    URL: https://github.com/conan-io/conan-center-index
    Homepage: https://github.com/fmtlib/fmt
    License: MIT
    Description: A safe and fast alternative to printf and IOStreams.
    Topics: conan, fmt, format, iostream, printf
    Recipe: Cache
    Binary: Download
    Binary remote: conan-center
    Creation date: 2020-01-10 05:49:25

conan info fmt/6.1.2@_/_ -o fmt:header_only=True
fmt/6.1.2
    ID: 5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9
    BuildID: None
    Remote: conan-center=https://conan.bintray.com
    URL: https://github.com/conan-io/conan-center-index
    Homepage: https://github.com/fmtlib/fmt
    License: MIT
    Description: A safe and fast alternative to printf and IOStreams.
    Topics: conan, fmt, format, iostream, printf
    Recipe: Cache
    Binary: Download
    Binary remote: conan-center
    Creation date: 2020-01-10 05:49:25
```

</details>

The setting for multiple_headers doesn't affect the package ID. When a package consumer tries to pull the package with multiple_headers set to true, conan downloads the "equivalent" package that was built with multiple_headers set to true. The only way I was able to get the multi-header version was to build locally.

I think the root cause is that the package_id method calls [self.info.header_only()](https://docs.conan.io/en/latest/reference/conanfile/methods.html#self-info-header-only) which produces a single package ID regardless of settings or options.
diff --git a/recipes/nlohmann_json/all/conanfile.py b/recipes/nlohmann_json/all/conanfile.py
index 860cb7528..7f31e2e2b 100644
--- a/recipes/nlohmann_json/all/conanfile.py
+++ b/recipes/nlohmann_json/all/conanfile.py
@@ -8,7 +8,6 @@ class NlohmannJsonConan(ConanFile):
     description = "JSON for Modern C++ parser and generator."
     topics = ("conan", "jsonformoderncpp", "nlohmann_json", "json", "header-only")
     url = "https://github.com/conan-io/conan-center-index"
-    settings = "os", "compiler", "arch", "build_type"
     no_copy_source = True
     license = "MIT"
     options = {
@@ -47,7 +46,3 @@ class NlohmannJsonConan(ConanFile):
             os.remove(os.path.join(self.package_folder, "nlohmann_json.natvis"))
         except FileNotFoundError:
             pass
-
-
-    def package_id(self):
-        self.info.header_only()
['recipes/nlohmann_json/all/conanfile.py']