Fixes the basketball and thunderdome baseturf issue. (#74461)

## About The Pull Request

Simply modifies the should_place_on_top value on these maps templates so
that they overwrite their baseturfs rather that creating an evergrowing
stack. I've also done the same for Mafia and CTF which should also not
be creating stacked baseturfs.
Fixes #69711
Should fix #74443 too since its the same issue.
## Why It's Good For The Game

Bugfix good.
## Changelog
🆑
fix: Basketball and Thunderdome maps should not load with broken turfs
are several resets of their maps.
/🆑

code/modules/admin/verbs/secrets.dm

@@ -143,6 +143,7 @@ GLOBAL_DATUM(everyone_a_traitor, /datum/everyone_is_a_traitor_controller)
 					qdel(obj) //Clear objects
 
 			var/datum/map_template/thunderdome_template = SSmapping.map_templates[THUNDERDOME_TEMPLATE_FILE]
+			thunderdome_template.should_place_on_top = FALSE
 			var/turf/thunderdome_corner = locate(thunderdome.x - 3, thunderdome.y - 1, 1) // have to do a little bit of coord manipulation to get it in the right spot
 			thunderdome_template.load(thunderdome_corner)