[MIRROR] Basic mobs don't become dense upon death [MDB IGNORE] (#18679)

* Basic mobs don't become dense upon death (#72554)

## About The Pull Request

In #72260 what was previously a var became a flag, which was a sensible
change, however this inverted the default behaviour.
In virtually all cases we want dead mobs to _stop_ being dense, this
added a requirement for the flag to be present for that to happen and
then didn't add the flag to any mobs.

Rather than add this to every mob I inverted the function of the flag.
My reasoning here is that _simple_ mobs seemingly never required this
behaviour, basic mobs are probably going to need it rarely if ever, and
including it in `basic_mob_flags` by default seems messy and easy to
leave off when setting other flags (plus #72524 implies to me we want to
avoid adding more default values).

Setting this manually on each mob seems kind of silly as a requirement
going forward and I can't think of a way we'd unit test for people
forgetting.

For the same reason I did the same thing with the
`STOP_ACTING_WHILE_DEAD` flag I added to the AI controller in a recent
PR, the flag should denote unusual behaviour not the default.

## Why It's Good For The Game

It looks really odd when you're constantly shuffling places with dead
mobs, they're not supposed to do that.
It's tedious to add `STOP_ACTING_WHILE_DEAD` to every AI controller when
that should be an obvious default assumption.

## Changelog

🆑
fix: Dead basic mobs are no longer "dense" objects and can be stepped
on.
/🆑

* Basic mobs don't become dense upon death

* Removes a flag we didn't need anymore.

* Forgot to remove this one

Co-authored-by: Jacquerel <hnevard@gmail.com>
Co-authored-by: GoldenAlpharex <58045821+GoldenAlpharex@users.noreply.github.com>
Co-authored-by: GoldenAlpharex <jerego1234@hotmail.com>

modular_skyrat/modules/basic_mobs/code/kiwi.dm

@@ -36,7 +36,7 @@
 /datum/ai_controller/basic_controller/kiwi
 	blackboard = list()
 
-	ai_traits = STOP_MOVING_WHEN_PULLED | STOP_ACTING_WHILE_DEAD
+	ai_traits = STOP_MOVING_WHEN_PULLED
 	ai_movement = /datum/ai_movement/basic_avoidance
 	idle_behavior = /datum/idle_behavior/idle_random_walk