remove: ActiveRecord::Relation::Methods#not

[rhiroe]

This method does not exist.

[github-actions]

@rhiroe Thanks for your contribution!

Please follow the instructions below for each change.
See also: https://github.com/ruby/gem_rbs_collection/blob/main/docs/CONTRIBUTING.md

Available commands

You can use the following commands by commenting on this PR.

• /merge: Merge this PR if CI passes

---

activerecord

You changed RBS files for an existing gem.
You need to get approval from the reviewers of this gem.

@hibariya, @ksss, @Little-Rubyist, please review this pull request.
If this change is acceptable, please make a review comment including APPROVE from here.

After that, the PR author or the reviewers can merge this PR.
Just comment /merge to merge this PR.

[ksss]

APPROVE
The #not method should be in WhereChain.

[rhiroe]

/merge