Removed some dead code from index removal.

This fixed the last remaining test errors and leaves us with only 11
failures.
Fixed two failing tests that needed to be updated.
Fixed another M2M test case.

Also removed the m2m_column_name and m2m_reverse_name accessors on
ManyToManyField since they are no longer used anywhere and won't have
well-defined semantics when multi-column ForeignKeys get implemented.