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/modules/shieldgen/emergency_shield.dm

@@ -120,7 +120,6 @@
 	density = 1
 	opacity = 0
 	anchored = 0
-	pressure_resistance = 2*ONE_ATMOSPHERE
 	req_access = list(access_engine)
 	var/const/max_health = 100
 	var/health = max_health