block: remove blk_init_flush() and its pair

Now mission of the two helpers is over, and just call
blk_alloc_flush_queue() and blk_free_flush_queue() directly.

Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --cc block/blk-core.c
Simple merge

diff --cc block/blk-flush.c
Simple merge

diff --cc block/blk-mq.c
Simple merge

diff --cc block/blk-sysfs.c
Simple merge

diff --cc block/blk.h
Simple merge