bcache: get rid of discard code from journal

bcache: get rid of discard code from journal

In bcache journal there is discard functionality but almost useless in
reality. Because discard happens after a journal bucket is reclaimed,
and the reclaimed bucket is allocated for new journaling immediately.
There is no time for underlying SSD to use the discard hint for internal
data management.

The discard code in bcache journal doesn't bring any performance
optimization and wastes CPU cycles for issuing discard bios. Therefore
this patch gits rid of it from journal.c and journal.h.

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