Fix memory leak in RageSurfaceUtils::Palettize

[mediancut( acolorhist_item *achv, int colors, int sum, int maxval, int newcolors )](https://github.com/stepmania/stepmania/edit/master/src/RageSurfaceUtils_Palettize.cpp#L326) returns a newly-allocated `acolorhist_item` which is stored as [`acolormap`](https://github.com/stepmania/stepmania/edit/master/src/RageSurfaceUtils_Palettize.cpp#L145)

However, that memory is never freed.