Moves /datum/var/signal_enabled to datum flags (#56372)

* Moves /datum/var/signal_enabled to datum flags

`signal_enabled` is a variable on /datum, so present almost every object
in the game. Folding it into the existing `datum_flags` variable will
save allocating a variable on every datum in the game.

- Clown weaponry was using the `signal_enabled` variable to turn their
  attached slippery component on and off when the shield/sword was
  toggled. They now just remove/add the component, rather than touching
  deep datum internals.

code/datums/datum.dm

@@ -36,12 +36,6 @@
 	var/list/comp_lookup
 	/// Lazy associated list in the structure of `signals:proctype` that are run when the datum receives that signal
 	var/list/list/datum/callback/signal_procs
-	/**
-	  * Is this datum capable of sending signals?
-	  *
-	  * Set to true when a signal has been registered
-	  */
-	var/signal_enabled = FALSE
 
 	/// Datum level flags
 	var/datum_flags = NONE
@@ -106,7 +100,7 @@
 		qdel(timer)
 
 	//BEGIN: ECS SHIT
-	signal_enabled = FALSE
+	datum_flags &= ~DF_SIGNAL_ENABLED
 
 	var/list/dc = datum_components
 	if(dc)