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/structures/grille.dm

@@ -6,7 +6,6 @@
 	density = 1
 	anchored = 1
 	flags = CONDUCT
-	pressure_resistance = 5*ONE_ATMOSPHERE
 	layer = 2.9
 	explosion_resistance = 1
 	var/health = 10

code/game/objects/structures/mop_bucket.dm

@@ -5,7 +5,6 @@
 	icon_state = "mopbucket"
 	density = 1
 	w_class = 3
-	pressure_resistance = 5
 	flags = OPENCONTAINER
 	var/amount_per_transfer_from_this = 5	//shit I dunno, adding this so syringes stop runtime erroring. --NeoFite
 

code/game/objects/structures/stool_bed_chair_nest/bed.dm

@@ -12,7 +12,6 @@
 	desc = "This is used to lie in, sleep in or strap on."
 	icon = 'icons/obj/furniture.dmi'
 	icon_state = "bed"
-	pressure_resistance = 15
 	anchored = 1
 	can_buckle = 1
 	buckle_dir = SOUTH

code/game/objects/structures/window.dm

@@ -6,7 +6,6 @@
 	w_class = 3
 
 	layer = 3.2//Just above doors
-	pressure_resistance = 4*ONE_ATMOSPHERE
 	anchored = 1.0
 	flags = ON_BORDER
 	var/maxhealth = 14.0

code/game/objects/structures/window_spawner.dm

@@ -9,7 +9,6 @@
 	icon_state = "wingrille"
 	density = 1
 	anchored = 1.0
-	pressure_resistance = 4*ONE_ATMOSPHERE
 	var/win_path = /obj/structure/window/basic
 	var/activated