Remove broken source tests

Introduced in 212f3c02521b993c9a88c3e9e80793d4b0db2b0b, unfortunately
testing this isn't possible because you can't commit a .git directory.
This means by luck the tests worked locally, and the `should return its
git url` test only worked because I personally checked out Core as
https://github.com/CocoaPods/Core since git inherits from parent
directories. This would be different for other users.

I'm removing these broken tests as I don't see any solution to fix them.