Removes unused pressure_resistance variable.

This is used with FEA not ZAS

Though it's kind of funny to look at what some of these values were set at.
(For reference: ONE_ATMOSPHERE = 101.325)

code/game/objects/items.dm

@@ -14,7 +14,6 @@
 	var/slot_flags = 0		//This is used to determine on which slots an item can fit.
 	var/no_attack_log = 0			//If it's an item we don't want to log attack_logs with, set this to 1
 	pass_flags = PASSTABLE
-	pressure_resistance = 5
 //	causeerrorheresoifixthis
 	var/obj/item/master = null