bcache: remove discard code from alloc.c

bcache: remove discard code from alloc.c

Bcache allocator initially has no free space to allocate. Firstly it
does a garbage collection which is triggered by a cache device write
and fills free space into ca->free[] lists. The discard happens after
the free bucket is handled by garbage collection added into one of the
ca->free[] lists. But normally this bucket will be allocated out very
soon to requester and filled data onto it. The discard hint on this
bucket LBA range doesn't help SSD control to improve internal erasure
performance, and waste extra CPU cycles to issue discard bios.

This patch removes the almost-useless discard code from alloc.c.

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