Revert "Restore 1.6 fix for generic relations."

This reverts commit 19288b896a5ccb146ae8fe8e25cde5a768079c0d.

_meta.get_all_field_names() load the app cache. This cannot be called
during the app cache loading.

Those line have been posing problems as seen in:

 * 19288b896a5ccb146ae8fe8e25cde5a768079c0d
 * d2b68151ca936422eef3d0b7cc2a8e63f5e2d4d1
 * 69acbfd8f025d2b245c4c8e8ca4d1484f0c1228d
Fix circular import problem and keep Django 1.6 compatibility

See:
 * f48390c4c8d61ca499f277f2ae1c2346262b949d
 * 19288b896a5ccb146ae8fe8e25cde5a768079c0d
 * d2b68151ca936422eef3d0b7cc2a8e63f5e2d4d1
 * 69acbfd8f025d2b245c4c8e8ca4d1484f0c1228d

Thank you Gavin Wahl (gwahl at fusionbox dot com) for the solution.