removes add_gas() and assert_gas(). next up: do further testing with garbage_collect() present and not present

code/game/mecha/equipment/tools/other_tools.dm

@@ -421,7 +421,6 @@
 	if(!istype(T))
 		return
 	var/datum/gas_mixture/GM = new
-	GM.add_gas(/datum/gas/plasma)
 	if(prob(10))
 		GM.gases[/datum/gas/plasma] += 100
 		GM.temperature = 1500+T0C //should be enough to start a fire

code/game/mecha/mecha.dm

@@ -237,7 +237,6 @@
 	cabin_air = new
 	cabin_air.temperature = T20C
 	cabin_air.volume = 200
-	cabin_air.add_gases(/datum/gas/oxygen, /datum/gas/nitrogen)
 	cabin_air.gases[/datum/gas/oxygen] = O2STANDARD*cabin_air.volume/(R_IDEAL_GAS_EQUATION*cabin_air.temperature)
 	cabin_air.gases[/datum/gas/nitrogen] = N2STANDARD*cabin_air.volume/(R_IDEAL_GAS_EQUATION*cabin_air.temperature)
 	return cabin_air