mbcache: Remove mb_cache_entry_delete()

Nobody uses mb_cache_entry_delete() anymore. Remove it.

Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220712105436.32204-9-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>