Development installation fails
After we updated `common` in #8368, the development installation doesn't work anymore:

```
$ inv docker.build
Traceback (most recent call last):
  File "/home/juanlu/.local/bin/inv", line 8, in <module>
    sys.exit(program.run())
  File "/home/juanlu/.local/pipx/venvs/invoke/lib/python3.8/site-packages/invoke/program.py", line 373, in run
    self.parse_collection()
  File "/home/juanlu/.local/pipx/venvs/invoke/lib/python3.8/site-packages/invoke/program.py", line 465, in parse_collection
    self.load_collection()
  File "/home/juanlu/.local/pipx/venvs/invoke/lib/python3.8/site-packages/invoke/program.py", line 696, in load_collection
    module, parent = loader.load(coll_name)
  File "/home/juanlu/.local/pipx/venvs/invoke/lib/python3.8/site-packages/invoke/loader.py", line 76, in load
    module = imp.load_module(name, fd, path, desc)
  File "/usr/lib/python3.8/imp.py", line 234, in load_module
    return load_source(name, filename, file)
  File "/usr/lib/python3.8/imp.py", line 171, in load_source
    module = _load(spec)
  File "<frozen importlib._bootstrap>", line 702, in _load
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 848, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/tmp/readthedocs.org/tasks.py", line 32, in <module>
    common.tasks.upgrade_all_packages,
AttributeError: module 'common.tasks' has no attribute 'upgrade_all_packages'
```
diff --git a/tasks.py b/tasks.py
index 710622d1c..6299cff25 100644
--- a/tasks.py
+++ b/tasks.py
@@ -27,13 +27,6 @@ namespace.add_collection(
     name='github',
 )
 
-namespace.add_collection(
-    Collection(
-        common.tasks.upgrade_all_packages,
-    ),
-    name='packages',
-)
-
 namespace.add_collection(
     Collection.from_module(
         common.dockerfiles.tasks,
['tasks.py']