avfilter/vf_waveform: add subsampling input support for remaining filters

Remove achroma filter, as same output can be done with lowpass filter
and multiple components with overlay display.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

doc/filters.texi

@@ -13171,9 +13171,6 @@ Similar as above, but shows difference between blue and red chroma.
 @item chroma
 Displays only chroma.
 
-@item achroma
-Similar as above, but shows difference between blue and red chroma.
-
 @item color
 Displays actual color value on waveform.