Assert that we aren't using most of "move_to_correct_build_directory"

Since self._ideal_build_dir is only either None or a string, we must
have always been returning from this function.

This goes back to pip 10.0.0, so this code has been dead for some time.

src/pip/_internal/req/req_install.py

@@ -412,8 +412,9 @@ class InstallRequirement(object):
             ])
         )
 
-        if self.source_dir is not None:
-            return
+        assert self.source_dir is not None
+
+        return
 
         assert self._temp_build_dir
         assert (