Project.has_valid_clone is not really used/useful

[humitos]

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 Add has_valid_clone and has_valid_webhook to ProjectAdminSerializer #4107)

[humitos]

@ericholscher can you take a look at this and decide if I should remove the field or make it useful?

[ericholscher]

I think it's useful data to have. We originally added it to have basic stats, and the ability to query projects that aren't hooked up to a valid git repo. So I think adding it is probably useful.