Removes an unused gas from the code

I have no idea why this is in the code.
Its unobtainable in game and would be difficult to create even with
admin powers, as there's no canister to spawn it in or anything like
that so you'd have to VV an already existing gas.

code/ATMOSPHERICS/components/trinary_devices/filter.dm

@@ -40,7 +40,7 @@
 	..()
 	switch(filter_type)
 		if(0) //removing hydrocarbons
-			filtered_out = list("phoron", "oxygen_agent_b")
+			filtered_out = list("phoron")
 		if(1) //removing O2
 			filtered_out = list("oxygen")
 		if(2) //removing N2
@@ -95,10 +95,10 @@
 
 /obj/machinery/atmospherics/trinary/filter/process()
 	..()
-	
+
 	last_power_draw = 0
 	last_flow_rate = 0
-	
+
 	if((stat & (NOPOWER|BROKEN)) || !use_power)
 		return