bcache: remove unused constants

Remove constants MAX_NEED_GC and MAX_SAVE_PRIO in btree.c that have been unused
since initial commit.

Signed-off-by: Robert Pang <robertpang@google.com>
Signed-off-by: Coly Li <colyli@kernel.org>
Link: https://lore.kernel.org/r/20250527051601.74407-3-colyli@kernel.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>