commit: Remove unused check

We support top == active for commit now, remove the check and add an
assertion here.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/block/commit.c b/block/commit.c
index d4090cb..acec4ac 100644
--- a/block/commit.c
+++ b/block/commit.c
@@ -198,13 +198,7 @@
         return;
     }
 
-    /* Once we support top == active layer, remove this check */
-    if (top == bs) {
-        error_setg(errp,
-                   "Top image as the active layer is currently unsupported");
-        return;
-    }
-
+    assert(top != bs);
     if (top == base) {
         error_setg(errp, "Invalid files for merge: top and base are the same");
         return;