Drop deactivated merger action

[kachick]

Since we moved the repository to this organization, the auto merger is failing due to different permissions. #139, #140

failed to create review: GraphQL: GitHub Actions is not permitted to approve pull requests. (addPullRequestReview)

So, I think just removing the action will be reasonable, considering the strict organizational policy.

Appendix: Allow some permissions to enable auto merger again

gh repo edit --enable-auto-merge

gh api \
  --method PUT \
  -H "Accept: application/vnd.github+json" \
  -H "X-GitHub-Api-Version: 2022-11-28" \
  '/repos/{owner}/{repo}/actions/permissions/workflow' \
  -F can_approve_pull_request_reviews=true

ref: https://github.com/orgs/mise-plugins/discussions/13

[kachick]

I still think same changes should be applied in https://github.com/mise-plugins/asdf-cargo-make and https://github.com/mise-plugins/asdf-zigmod

cc: @mise-plugins