Remove unused files from extras

[Vladislav4KZ]

fastdl.txt - removed dead server.

gameinfo.txt - not used, as it's autogenerated in gamedir.
sound/bot/null.wav - used to be used as a stub for bot chatter. No longer used.

gfx/shell/colors.lst - Counter-Strike doesn't use custom colors.

Unused shell textures:

gfx/shell/divider.bmp
gfx/shell/dnarrowf.bmp
gfx/shell/dnarrowp.bmp
gfx/shell/down.bmp
gfx/shell/favorite.bmp
gfx/shell/head_createroom.bmp
gfx/shell/head_filter.bmp
gfx/shell/head_gore.bmp
gfx/shell/head_keyboard.bmp
gfx/shell/head_readme.bmp
gfx/shell/head_room.bmp
gfx/shell/head_rooms.bmp
gfx/shell/head_specgames.bmp
gfx/shell/larrowflyover.bmp
gfx/shell/larrowpressed.bmp
gfx/shell/rarrowflyover.bmp
gfx/shell/rarrowpressed.bmp
gfx/shell/sm_dnarf.bmp
gfx/shell/uparrowf.bmp
gfx/shell/uparrowp.bmp