Cleanup: remove the game_state parameter to game_colours(). No game

Cleanup: remove the game_state parameter to game_colours(). No game
was actually using it, and also it wasn't being called again for
different game states or different game parameters, so it would have
been a mistake to depend on anything in that game state. Games are
now expected to commit in advance to a single fixed list of all the
colours they will ever need, which was the case in practice already
and simplifies any later port to a colour-poor platform. Also this
change has removed a lot of unnecessary faff from midend_colours().

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@6416 cda61777-01e9-0310-a592-d414129be87e