Makes tendrils less out for blood, cleans up The Thing's code (#96332)

## About The Pull Request

I didn't realize that default aggro range for mobs is 9, so tendrils
would try to attack you with their whips and spikes despite you being
completely out of their range, which looks pretty funny.
I've fixed this by separating out aggro ***grab*** range and aggro
sustain ranges, lowering latter to 5 for tendrils, and adding checks for
target distance for both abilities it has. The Thing also had a similar
behavior but used rather jank code for that (which I don't think even
worked), so I've modified it to use the new system as well.

## Why It's Good For The Game

Stationary mobs probably shouldn't try to attack mobs completely out of
their reach

## Changelog
🆑
fix: Tendrils no longer try to attack you when you're out of their reach
code: Cleaned up The Thing's aggro code
/🆑

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

@@ -22,9 +22,5 @@
 /datum/ai_planning_subtree/simple_find_target/to_flee
 	target_key = BB_BASIC_MOB_FLEE_TARGET
 
-/datum/ai_planning_subtree/simple_find_target/increased_range
-	target_behavior = /datum/ai_behavior/find_potential_targets/bigger_range
-
 /datum/ai_planning_subtree/simple_find_target/hunt
 	strategy_key = BB_HUNT_TARGETING_STRATEGY
-