dm cache: Remove unused btracker_nr_writebacks_queued

btracker_nr_writebacks_queued() has been unused since commit
2e63309507c8 ("dm cache policy smq: don't do any writebacks unless IDLE")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <[e-mail]>
Signed-off-by: Mikulas Patocka <[e-mail]>

drivers/md/dm-cache-background-tracker.c

···
 	}
 }
 
-unsigned int btracker_nr_writebacks_queued(struct background_tracker *b)
-{
-	return atomic_read(&b->pending_writebacks);
-}
-EXPORT_SYMBOL_GPL(btracker_nr_writebacks_queued);
-
 unsigned int btracker_nr_demotions_queued(struct background_tracker *b)
 {
 	return atomic_read(&b->pending_demotes);

drivers/md/dm-cache-background-tracker.h

···
  */
 void btracker_destroy(struct background_tracker *b);
 
-unsigned int btracker_nr_writebacks_queued(struct background_tracker *b);
 unsigned int btracker_nr_demotions_queued(struct background_tracker *b);
 
 /*