dm cache: Remove unused btracker_nr_writebacks_queued

btracker_nr_writebacks_queued() has been unused since commit
2e633095 ("dm cache policy smq: don't do any writebacks unless IDLE")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>