partial fix for #69624

partial fix for #69624

This uncomments a call to _delete_empty_auto_increment.  True to its name,
this function removes NULL auto-increments from the column info so that
they are not INSERTed.  The call to this method was commented out in
93ad3243, but I cannot find anyone who remembers why, and re-enabling the
call does not break any other tests on postgres, mysql, or sqlite.

However, this change alone does not cause the test to pass; there are still
issues with databases which enforce foreign constraints (tested on
postgres, mysql, and sqlite with the foreign_keys pragma).  I'm not sure if
the test itself is broken, but maybe it should at least be marked as TODO.