[lua] Fixes to chigoe mixins

[MowFord]

spawn on mobskill behavior in various zones
do not drop loot when instagib happens

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• I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• I have read and understood the Contributing Guide and the Code of Conduct.
• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

The chigoe mixin for spawning chigoe pets on Marid mobskills had a few issues:

• they didn't spawn engaged
• they never went away
• they had duplicate code (every instance of a chigoe being spawned by this chigoe_pet mixin already has a Chigoe.lua that defines the instagib behavior)

This PR resolves those

Separately, the chigoe mixin did not properly set no_drop on the mobs when they are set to not give xp

Steps to test these changes

Go to the zones that have chigoe_pet mixin mobs (no idea why that diremite has the mixin, afaict it isn't supposed to spawn chigoes on mobskills?).

Confirm when they mobskill a chigoe spawns and immediately aggros/has proper chigoe behavior

https://imgur.com/nxVM12q