Remove unused function drbd_bm_write_lazy() in drbd/drbd_bitmap.c.

This eliminates the following warning in drbd/drbd_bitmap.c:
drivers/block/drbd/drbd_bitmap.c:1208:5: warning: no previous prototype for ‘drbd_bm_write_lazy’ [-Wmissing-prototypes]
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>