Removes watcher overwatch ability (#78292)

## About The Pull Request

Removes the "overwatch" ability from Watchers, allowing them to use
their "look away" ability at any health threshold instead (but only if
it's been fighting you for at least 5 seconds or if you attack it).
Drops the cooldown on the gaze a little bit to compensate.

Also fixes some weird behaviour I noticed while testing:
- It won't cancel its own ability by trying to back away from you.
- It will look at you when it shoots you.

## Why It's Good For The Game

I was cooking too hard with this one.
- Two abilities overcomplicates what are supposed to be a pretty simple
mob you fight in packs.
- It wasn't obvious what you were actually supposed to do when
targetted.
- Doing it wrong could be very punishing in groups.
- Doing it _right_ was still kind of unexciting.

This is an ability to give to an elite, not a random trash mob.

## Changelog

🆑
balance: Watchers will no longer put you at gunpoint.
/🆑

code/datums/ai/basic_mobs/basic_subtrees/maintain_distance.dm

@@ -11,6 +11,9 @@
 
 /datum/ai_planning_subtree/maintain_distance/SelectBehaviors(datum/ai_controller/controller, seconds_per_tick)
 	. = ..()
+	var/mob/living/living_pawn = controller.pawn
+	if(LAZYLEN(living_pawn.do_afters))
+		return
 	var/atom/target = controller.blackboard[target_key]
 	if (!isliving(target) || !can_see(controller.pawn, target, view_distance))
 		return // Don't run away from cucumbers, they're not snakes

code/datums/ai/basic_mobs/basic_subtrees/use_mob_ability.dm

@@ -4,7 +4,7 @@
  */
 /datum/ai_planning_subtree/use_mob_ability
 	/// Blackboard key for the ability
-	var/ability_key
+	var/ability_key = BB_GENERIC_ACTION
 	/// Behaviour to perform using ability
 	var/use_ability_behaviour = /datum/ai_behavior/use_mob_ability
 	/// If true we terminate planning after trying to use the ability.