lavfi: remove vf_slicify

The following commit will make it useless.

The crop_scale_vflip FATE test changes because of off-by-one differences
in output when vflipped slices are passed to sws.

libavfilter/allfilters.c

@@ -85,7 +85,6 @@ void avfilter_register_all(void)
     REGISTER_FILTER (SETSAR,      setsar,      vf);
     REGISTER_FILTER (SETTB,       settb,       vf);
     REGISTER_FILTER (SHOWINFO,    showinfo,    vf);
-    REGISTER_FILTER (SLICIFY,     slicify,     vf);
     REGISTER_FILTER (SPLIT,       split,       vf);
     REGISTER_FILTER (TRANSPOSE,   transpose,   vf);
     REGISTER_FILTER (UNSHARP,     unsharp,     vf);