Remove non-global references to SSgameMaster and SSevent_ticker

- Best practice to avoid holding onto references to the subsystem instances aside from the global variables. (In case of restart/recover etc).
- The reference to GM was entirely unused by any event or meta-event.  /datum/game_master would work fine using the global refs.

code/datums/game_masters/default.dm

@@ -71,12 +71,12 @@
 // If no list is passed, all the events will be returned.
 /datum/game_master/default/proc/filter_events_by_departments(list/departments)
 	. = list()
-	for(var/E in ticker.available_events)
+	for(var/E in SSgame_master.available_events)
 		var/datum/event2/meta/event = E
 		if(!event.enabled)
 			continue
 		if(event.chaotic_threshold && !ignore_round_chaos)
-			if(ticker.danger > event.chaotic_threshold)
+			if(SSgame_master.danger > event.chaotic_threshold)
 				continue
 		// An event has to involve all of these departments to pass.
 		var/viable = TRUE

code/datums/game_masters/other_game_masters.dm

@@ -3,7 +3,7 @@
 // * Does not react to danger at all.
 /datum/game_master/classic/choose_event()
 	var/list/weighted_events = list()
-	for(var/E in ticker.available_events)
+	for(var/E in SSgame_master.available_events)
 		var/datum/event2/meta/event = E
 		if(!event.enabled)
 			continue
@@ -20,7 +20,7 @@
 // * Has no goals, and instead chooses randomly, ignoring weights.
 // * Does not react to danger at all.
 /datum/game_master/super_random/choose_event()
-	return pick(ticker.available_events)
+	return pick(SSgame_master.available_events)
 
 
 // The `brutal` game master tries to run dangerous events frequently.
@@ -31,7 +31,7 @@
 
 /datum/game_master/brutal/choose_event()
 	var/list/weighted_events = list()
-	for(var/E in ticker.available_events)
+	for(var/E in SSgame_master.available_events)
 		var/datum/event2/meta/event = E
 		if(!event.enabled)
 			continue