Renames delta time to be a more obvious name (#74654)

This tracks the seconds per tick of a subsystem, however note that it is
not completely accurate, as subsystems can be delayed, however it's
useful to have this number as a multiplier or ratio, so that if in
future someone changes the subsystem wait time code correctly adjusts
how fast it applies effects

regexes used

git grep --files-with-matches --name-only 'DT_PROB' | xargs -l sed -i
's/DT_PROB/SPT_PROB/g'
git grep --files-with-matches --name-only 'delta_time' | xargs -l sed -i
's/delta_time/seconds_per_tick/g'

code/modules/assembly/timer.dm

@@ -63,10 +63,10 @@
 		timing = TRUE
 	update_appearance()
 
-/obj/item/assembly/timer/process(delta_time)
+/obj/item/assembly/timer/process(seconds_per_tick)
 	if(!timing)
 		return
-	time -= delta_time
+	time -= seconds_per_tick
 	if(time <= 0)
 		timing = FALSE
 		timer_end()