block: Drop backing_bs()

We want to make it explicit where bs->backing is used, and we have done
so.  The old role of backing_bs() is now effectively taken by
bdrv_cow_bs().

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 8205cca..bb3d1c8 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -1003,11 +1003,6 @@
     MIRROR_LEAVE_BACKING_CHAIN,
 } BlockMirrorBackingMode;
 
-static inline BlockDriverState *backing_bs(BlockDriverState *bs)
-{
-    return bs->backing ? bs->backing->bs : NULL;
-}
-
 
 /* Essential block drivers which must always be statically linked into qemu, and
  * which therefore can be accessed without using bdrv_find_format() */