bcache: remove retry_flush_write from struct cache_set

In struct cache_set, retry_flush_write is added for commit c4dc2497
("bcache: fix high CPU occupancy during journal") which is reverted in
previous patch.

Now it is useless anymore, and this patch removes it from bcache code.

Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>