Turn /datum.isprocessing into a datum flag (#38193)

code/controllers/subsystem/machines.dm

@@ -45,7 +45,7 @@ SUBSYSTEM_DEF(machines)
 		else
 			processing -= thing
 			if (!QDELETED(thing))
-				thing.isprocessing = FALSE
+				thing.datum_flags &= ~DF_ISPROCESSING
 		if (MC_TICK_CHECK)
 			return
 

code/controllers/subsystem/processing/processing.dm

@@ -27,7 +27,6 @@ SUBSYSTEM_DEF(processing)
 		if (MC_TICK_CHECK)
 			return
 
-/datum/var/isprocessing = FALSE
 /datum/proc/process()
 	set waitfor = 0
 	STOP_PROCESSING(SSobj, src)