Unify datum var definitions

Inspired by https://github.com/tgstation/tgstation/pull/29636
Also consolidated some sideways overridden /datum/Delete() here to reduce proc-call overhead.

code/datums/weakref.dm

@@ -1,10 +1,3 @@
-/datum
-	var/weakref/weakref
-
-/datum/Destroy()
-	weakref = null // Clear this reference to ensure it's kept for as brief duration as possible.
-	. = ..()
-
 //obtain a weak reference to a datum
 /proc/weakref(datum/D)
 	if(!istype(D))