Remove hexadecimal color names from palette files

There is some fallback code in _loadPaletteFile that formats a
hexadecimal representation from the color value if the name is empty,
therefore the application behavior is not changed. However, this avoids
looking up translations for these names, which improves startup time by
about 300 ms on my Windows machine. Colors with regular names
(like 'blue') are not affected by this change and still properly
translated.

The following command was run inside share/palettes/ to generate this commit:

sed -i -E "s/(^\s*[0-9]+\s+[0-9]*\s+[0-9]*)\s+#?[0-9A-Fa-f]{6}/\1/" *.gpl