Removes oxygen_agent_b and volatile_fuel gases

- oxygen_agent_b wasn't used at all, except for filters which, for some reason filtered it in addition to phoron
- volatile_fuel was only used in two chemistry reactions as reaction result, it was replaced with gaseous phoron.

code/modules/reagents/Chemistry-Reagents/Chemistry-Reagents-Toxins.dm

@@ -58,7 +58,7 @@
 /datum/reagent/toxin/phoron/touch_turf(var/turf/simulated/T)
 	if(!istype(T))
 		return
-	T.assume_gas("volatile_fuel", volume, T20C)
+	T.assume_gas("phoron", volume, T20C)
 	remove_self(volume)
 
 /datum/reagent/toxin/cyanide //Fast and Lethal

code/modules/reagents/Chemistry-Recipes.dm

@@ -667,7 +667,7 @@
 /datum/chemical_reaction/napalm/on_reaction(var/datum/reagents/holder, var/created_volume)
 	var/turf/location = get_turf(holder.my_atom.loc)
 	for(var/turf/simulated/floor/target_tile in range(0,location))
-		target_tile.assume_gas("volatile_fuel", created_volume, 400+T0C)
+		target_tile.assume_gas("phoron", created_volume, 400+T0C)
 		spawn (0) target_tile.hotspot_expose(700, 400)
 	holder.del_reagent("napalm")
 	return