block: bfq: Remove an unused function definition

This patch is the result of the analysis of a sparse report.

Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/20220617204433.102022-1-bvanassche@acm.org