[NO GBP] Rabbits won't become dense when they die (#71499)

## About The Pull Request

Made a last minute suggestion change to a PR which was just merged and
then like a couple minutes after that we decided it was a bad idea
actually.
This implements the _actual_ default behaviour we expect mobs to have.
Dense mobs become undense when they die. Mobs which already aren't dense
continue not being dense. They return to their original state when
they're alive.

## Why It's Good For The Game

This is largely how you expect corpses to work,

## Changelog
🆑
fix: Rabbits don't suddenly become more dense when killed than they were
in life.
/🆑

code/modules/mob/living/basic/basic.dm

@@ -77,8 +77,8 @@
 	var/icon_gib = null
 	///Flip the sprite upside down on death. Mostly here for things lacking custom dead sprites.
 	var/flip_on_death = FALSE
-	///Invert the mob's density on death. A usually dense mob will become passable, a passable one will become dense.
-	var/flip_density_on_death = TRUE
+	///Removes density upon death, restores the original state if alive.
+	var/become_passable_on_death = TRUE
 
 	///If the mob can be spawned with a gold slime core. HOSTILE_SPAWN are spawned with plasma, FRIENDLY_SPAWN are spawned with blood.
 	var/gold_core_spawnable = NO_SPAWN
@@ -122,8 +122,8 @@
 		icon_state = icon_dead
 		if(flip_on_death)
 			transform = transform.Turn(180)
-		if (flip_density_on_death)
-			set_density(!density)
+		if (become_passable_on_death)
+			set_density(FALSE)
 
 /mob/living/basic/revive(full_heal_flags = NONE, excess_healing = 0, force_grab_ghost = FALSE)
 	. = ..()
@@ -132,8 +132,8 @@
 	icon_state = icon_living
 	if (flip_on_death)
 		transform = transform.Turn(180)
-	if (flip_density_on_death)
-		set_density(!density)
+	if (become_passable_on_death)
+		set_density(initial(density))
 
 /mob/living/basic/proc/melee_attack(atom/target)
 	src.face_atom(target)