mint-arena: Only need one cg vmCvar for ui_singlePlayerActive

Don't make cg_singlePlayer MISSIONPACK only. Q3 code uses, not used in cgame yet
but it's annoying to have to move it outside MISSIONPACK in other games.

Leave ui_singlePlayerActive vmCvar_t in missionpack UI, for now at least.
Merge remote-tracking branch 'upstream/master'