block: Remove unused blk_pm_*() function definitions

Commit a1ce35fa ("block: remove dead elevator code") removed the last
callers of blk_pm_requeue_request(), blk_pm_add_request() and
blk_pm_put_request(). Hence remove the definitions of these functions.
Removing these functions removes all users of the struct request nr_pending
member. Hence also remove 'nr_pending'. Note: 'nr_pending' is no longer
used since commit 7cedffec ("block: Make blk_get_request() block for
non-PM requests while suspended").

Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>