lavfi: remove double .priv_class initializers

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/af_aformat.c

@@ -145,5 +145,4 @@ AVFilter avfilter_af_aformat = {
 
     .inputs        = avfilter_af_aformat_inputs,
     .outputs       = avfilter_af_aformat_outputs,
-    .priv_class    = &aformat_class,
 };

libavfilter/vf_blackframe.c

@@ -133,6 +133,4 @@ AVFilter avfilter_vf_blackframe = {
     .inputs    = avfilter_vf_blackframe_inputs,
 
     .outputs   = avfilter_vf_blackframe_outputs,
-
-    .priv_class = &blackframe_class,
 };

libavfilter/vf_crop.c

@@ -348,5 +348,4 @@ AVFilter avfilter_vf_crop = {
 
     .inputs    = avfilter_vf_crop_inputs,
     .outputs   = avfilter_vf_crop_outputs,
-    .priv_class = &crop_class,
 };