Moves ui references from the tgui subsystem to datums themselves (#76215)

## About The Pull Request

Unused vars have 0 memory cost, and the ref and list lookup here is
REALLY expensive, for both init and foam spreading.

## Why It's Good For The Game

Saves 0.2s off a station flood on meta, and 0.17s off init. More time in
other qdel heavy areas
Pulled off #76104 for the sake of cleanliness

code/modules/tgui/external.dm

@@ -70,7 +70,7 @@
  * change static data.
  */
 /datum/proc/update_static_data_for_all_viewers()
-	for (var/datum/tgui/window as anything in SStgui.open_uis_by_src[REF(src)])
+	for (var/datum/tgui/window as anything in open_uis)
 		window.send_full_update()
 
 /**