fate/scale2ref_keep_aspect: switch to vf_scale ref_*

tests/filtergraphs/scale2ref_keep_aspect

@@ -1,5 +1,4 @@
 sws_flags=+accurate_rnd+bitexact;
 testsrc=size=320x240 [main];
 testsrc=size=640x360 [ref];
-[main][ref] scale2ref=iw/4:ow/mdar [main][ref];
-[ref] nullsink
+[main][ref] scale=rw/4:ow/dar [main]