Kills spec_updatehealth, replaces it with use of COMSIG_LIVING_HEALTH_UPDATE (#78772)

## About The Pull Request

Quick refactor, replaces `spec_updatehealth` with use of
`COMSIG_LIVING_HEALTH_UPDATE`.

`spec_updatehealth` was only used by one species, ethereals, and it was
used in a (in my opinion) janky way by calling `spec_updatehealth`
constantly.

This has one downside - `COMSIG_LIVING_HEALTH_UPDATE` is NOT sent if the
mob is in godmode, while `spec_updatehealth` was, which means godmoded
ethereals may not update 100% accurately according to their health
before entering godmode, but I doubt this will come into play often at
all.

- [x] I tested this PR

## Why It's Good For The Game

`updatehealth` is kinda hot, not turbo hot but a little hot, so this
helps performance a bit.

## Changelog

🆑 Melbert
refactor: Refactors how ethereals update their color when damaged. 
/🆑

code/modules/admin/create_mob.dm

@@ -28,6 +28,6 @@
 	human.dna.species.randomize_active_underwear_only(human)
 	// Needs to be called towards the end to update all the UIs just set above
 	human.dna.initialize_dna(newblood_type = random_blood_type(), create_mutation_blocks = randomize_mutations, randomize_features = TRUE)
-	// Snowflake stuff (ethereals)
-	human.dna.species.spec_updatehealth(human)
+	// Snowflake for Ethereals
+	human.updatehealth()
 	human.updateappearance(mutcolor_update = TRUE)