Removes duplicate definitions of /datum/gamemode on almost every gamemode file (#25219)

* Why did every gamemode do this :(

* Contra request

code/modules/mob/living/carbon/superheroes.dm

@@ -1,8 +1,3 @@
-/datum/game_mode
-	var/list/datum/mind/superheroes = list()
-	var/list/datum/mind/supervillains = list()
-	var/list/datum/mind/greyshirts = list()
-
 /datum/superheroes
 	var/name
 	var/desc

code/modules/response_team/ert.dm

@@ -4,9 +4,6 @@
 #define ERT_TYPE_RED		2
 #define ERT_TYPE_GAMMA		3
 
-/datum/game_mode
-	var/list/datum/mind/ert = list()
-
 GLOBAL_LIST_EMPTY(response_team_members)
 GLOBAL_VAR_INIT(responseteam_age, 21) // Minimum account age to play as an ERT member
 GLOBAL_DATUM(active_team, /datum/response_team)