row_upd_index_is_referenced(), row_upd_check_references_constraints(),
row_purge_parse_undo_rec(): Remove duplicated code that calls
row_mysql_unfreeze_data_dictionary().

row_undo_ins_remove_clust_rec():
Assert that the data dictionary is X-latched.  Do not attempt to re-latch
the data dictionary, because doing so introduces race conditions.  This
re-latching was probably made unnecessary already in r1676, which keeps
the data dictionary X-latched in ha_innobase::add_index() for the complete
duration of the data dictionary operations.

row_undo_ins_parse_undo_rec(): Do not attempt to acquire the data dictionary
latch.  The data dictionary should be already X-latched when a transaction
that modifies system tables is rolled back.  This also holds in the rollback
during crash recovery: trx_rollback_active() will X-latch the data dictionary
when needed.