iris: Remove render cache hash table-based synchronization.

iris: Remove render cache hash table-based synchronization.

The render cache hash table is now *mostly* redundant with the more
general seqno matrix-based cache tracking mechanism.  Most hash table
operations are now gone except for the format mismatch checks done in
iris_cache_flush_for_render().  Redundant code removed as a separate
patch for bisectability.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3875>