Revert "Nanoui now supports use of multiple templates (#5105)"

This reverts commit a2841ce9d6.
It turns out nanoui already supports multiple templates.

code/modules/nano/nanoui.dm

@@ -519,8 +519,3 @@ nanoui is used to open and update nano browser uis
   */
 /datum/nanoui/proc/update(var/force_open = 0)
 	src_object.ui_interact(user, ui_key, src, force_open, master_ui, state)
-
-/datum/nanoui/proc/append_template(var/key, var/filename)
-	add_template(key, filename)
-	open()
-	update(1)