Makes mutations clean up after themselves (#80107)

## About The Pull Request

They weren't doing this before. This was an issue because they hold refs
to a `mob/living/carbon/owner` as well as a `datum/dna`, making it a
potential source of hard dels.

As far as I can tell, I do not see this causing any issues as
`remove_mutation(mutation)` takes a type path as an arg rather than a
reference, same thing with `get_mutation(mutation)`. I couldn't find any
examples of a reference to a mutation being reused by anything after
being removed.

---

When I was investigating potential reasons for why it might have been
like this I found more problem code. Timed dna injectors were setting
`target` to the return value of `add_mutation()`/`remove_mutation()`
which is a bool. This made no sense and would cause runtimes as well as
mislead people into thinking that the return value of those procs was a
`mob/living/carbon`.

## Why It's Good For The Game

Fixes an oversight, and headache further down the line.

## Changelog

🆑
fix: fixed mutations holding onto refs after removal
fix: fixes timed dna injectors
/🆑

code/datums/mutations/_mutations.dm

@@ -86,6 +86,12 @@
 		copy_mutation(copymut)
 	update_valid_chromosome_list()
 
+/datum/mutation/human/Destroy()
+	power_path = null
+	dna = null
+	owner = null
+	return ..()
+
 /datum/mutation/human/proc/on_acquiring(mob/living/carbon/human/acquirer)
 	if(!acquirer || !istype(acquirer) || acquirer.stat == DEAD || (src in acquirer.dna.mutations))
 		return TRUE
@@ -137,11 +143,7 @@
 		mut_overlay.Remove(get_visual_indicator())
 		owner.overlays_standing[layer_used] = mut_overlay
 		owner.apply_overlay(layer_used)
-	if(power_path)
-		// Any powers we made are linked to our mutation datum,
-		// so deleting ourself will also delete it and remove it
-		// ...Why don't all mutations delete on loss? Not sure.
-		qdel(src)
+	qdel(src)
 
 /mob/living/carbon/proc/update_mutations_overlay()
 	return

code/datums/mutations/body.dm

@@ -208,7 +208,7 @@
 	. = owner.monkeyize()
 
 /datum/mutation/human/race/on_losing(mob/living/carbon/human/owner)
-	if(owner && owner.stat != DEAD && (owner.dna.mutations.Remove(src)) && ismonkey(owner))
+	if(!QDELETED(owner) && owner.stat != DEAD && (owner.dna.mutations.Remove(src)) && ismonkey(owner))
 		owner.fully_replace_character_name(null, original_name)
 		. = owner.humanize(original_species)