Fix Unnesting Mob Density (#4134)

# About the pull request

This PR fixes issues where mobs that get unnested can sometimes have the
incorrect density value (allowing xenos to push them even if they aren't
standing or are dead).

Let me know if there are other conditions that should also be checked.

# Explain why it's good for the game

Fixes issues such as this one (same for dieing in nest): 


https://github.com/cmss13-devs/cmss13/assets/76988376/4daa4400-2fdd-4edb-be9b-a876cd895fe1

# Testing Photographs and Procedure
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<summary>Screenshots & Videos</summary>

Put screenshots and videos here with an empty line between the
screenshots and the `<details>` tags.

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# Changelog
:cl: Drathek
fix: Fixed unnested mobs sometimes having incorrect density allowing
xenos to push them
/:cl:

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

@@ -20,8 +20,6 @@
 	var/force_nest = FALSE
 	/// counterpart to buckling_y --> offsets the buckled mob when it buckles
 	var/list/buckling_x
-	/// saves the density of the buckled_mob
-	var/buckled_mob_density
 
 /obj/structure/bed/nest/Initialize(mapload, hive)
 	. = ..()
@@ -52,7 +50,6 @@
 		resisting_ready = FALSE
 
 	if(buckled_mob == current_mob)
-		buckled_mob_density = current_mob.density
 		current_mob.pixel_y = buckling_y["[dir]"]
 		current_mob.pixel_x = buckling_x["[dir]"]
 		current_mob.dir = turn(dir, 180)
@@ -70,7 +67,7 @@
 
 	current_mob.pixel_y = initial(buckled_mob.pixel_y)
 	current_mob.pixel_x = initial(buckled_mob.pixel_x)
-	current_mob.density = buckled_mob_density
+	current_mob.density = !(current_mob.lying || current_mob.stat == DEAD)
 	if(dir == SOUTH)
 		current_mob.layer = initial(current_mob.layer)
 		if(!ishuman(current_mob))
@@ -281,8 +278,6 @@
 	buckled_mob.old_y = 0
 	REMOVE_TRAIT(buckled_mob, TRAIT_NESTED, TRAIT_SOURCE_BUCKLE)
 	var/mob/living/carbon/human/buckled_human = buckled_mob
-	if(buckled_human.stat == DEAD )
-		buckled_mob_density = FALSE
 
 	var/mob/dead/observer/G = ghost_of_buckled_mob
 	var/datum/mind/M = G?.mind
@@ -311,7 +306,6 @@
 
 /obj/structure/bed/nest/proc/healthcheck()
 	if(health <= 0)
-		buckled_mob_density = FALSE
 		deconstruct()
 
 /obj/structure/bed/nest/fire_act()