Project.has_valid_clone is not really used/useful
While debugging a different issue (https://github.com/readthedocs/readthedocs-corporate/issues/332) I found that `Project.has_valid_clone` is present in just 3 places in the whole codebase and it's not really used/useful.

Besides, we making an extra API call (https://github.com/rtfd/readthedocs.org/blob/10b6d53cbcdb33d9241a885e730784ba13da35e8/readthedocs/projects/tasks.py#L404) to set this to `True` on each build triggered after the setup step is done but since it's not present in the serializer (https://github.com/rtfd/readthedocs.org/blob/10b6d53cbcdb33d9241a885e730784ba13da35e8/readthedocs/restapi/serializers.py#L47) we are _always_ making that PUT request which in fact it doesn't update anything because that value is lost.

I did a quick query in production and there is 0 project with a valid clone :)

```
In [1]: Project.objects.filter(has_valid_clone=True).count()
Out[1]: 0
```

So, 

1. we can remove this field from our database without problem and also the related code
2. we can add this field to the `ProjectAdminSerializer` and make it happen (PR #4107)
diff --git a/readthedocs/restapi/serializers.py b/readthedocs/restapi/serializers.py
index b4d8b2b9b..97f4335f1 100644
--- a/readthedocs/restapi/serializers.py
+++ b/readthedocs/restapi/serializers.py
@@ -60,6 +60,8 @@ class ProjectAdminSerializer(ProjectSerializer):
             'requirements_file',
             'python_interpreter',
             'features',
+            'has_valid_clone',
+            'has_valid_webhook',
         )
 
 
diff --git a/readthedocs/rtd_tests/tests/test_api.py b/readthedocs/rtd_tests/tests/test_api.py
index 726ec8b06..743b25e90 100644
--- a/readthedocs/rtd_tests/tests/test_api.py
+++ b/readthedocs/rtd_tests/tests/test_api.py
@@ -735,6 +735,8 @@ class APIVersionTests(TestCase):
                 'enable_epub_build': True,
                 'enable_pdf_build': True,
                 'features': ['allow_deprecated_webhooks'],
+                'has_valid_clone': False,
+                'has_valid_webhook': False,
                 'id': 6,
                 'install_project': False,
                 'language': 'en',
['readthedocs/rtd_tests/tests/test_api.py', 'readthedocs/restapi/serializers.py']