avfilter/palette{gen,use}: revert support palettes with alpha

This reverts commit dea673d0d548c864ec85f9260d8900d944ef7a2a.

This change cannot work for several reasons, the most obvious ones are:

- the alpha is being part of the scoring of the color difference, even
  though we can not interpret the alpha as part of the perception of the
  color (we don't even know if it's premultiplied or postmultiplied)
- the colors are averaged with their alpha value which simply cannot
  work

The command proposed in the original thread of the patch actually
produces a completely broken file:

    ffmpeg -y -loglevel verbose -i fate-suite/apng/o_sample.png -filter_complex "split[split1][split2];[split1]palettegen=max_colors=254:use_alpha=1[pal1];[split2][pal1]paletteuse=use_alpha=1" -frames:v 1 out.png

We can see that many color pixels are off, but more importantly some
colors have a random alpha value: https://imgur.com/eFQ2UK7

I don't see any easy fix for this unfortunately, the approach appears to
be flawed by design.

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