Fix #4560 - file-xpm saving unused transparency
The XPM export plugin was setting the color None (transparency) on the
exported image palette for all images with an alpha channel, even if the
color was not used on any pixel.
Since it's nice to have None as the first color on the palette, mapped
to character ' ', and it could be too wasteful to scan all pixels twice,
the approach taken was to just undo it's premature insertion.