[Bug]: Remove post.instance_id column

[dessalines]

Requirements

• Is this a bug report? For questions or discussions use https://lemmy.ml/c/lemmy_support or the matrix chat.
• Did you check to see if this issue already exists?
• Is this only a single bug? Do not put multiple bugs in one issue.
• Do you agree to follow the rules in our Code of Conduct?
• Is this a backend issue? Use the lemmy-ui repo for UI / frontend issues.

Summary

The post's instance_id column is a dupe of the community's instance_id column, and its also not being set correctly.

It should be removed.

Steps to Reproduce

NA

Technical Details

NA

Version

main

Lemmy Instance URL

all

[Nutomic]

This was moved over from post_aggregates, iirc it was meant to optimize some sql queries. The logic to initialize it may have been lost in #5407. Using community.instance_id makes sense as its more explicit, otherwise its not clear whether you are getting that or getting person.instance_id.