Cannot disable jekyll-feed

### Before submitting an issue, please be sure to

- [X] Read [the contributing instructions](https://github.com/github/pages-gem/blob/master/docs/CONTRIBUTING.md#contributing-to-the-github-pages-gem)
- [X] Update to the latest Gem version (run `bundle update github-pages`)

### This issue affects

- [ ] The site generated by GitHub Pages
- [X] Building sites locally

*The GitHub Pages Gem is intended to help users replicate the GitHub Pages build environment locally. If your issue affects both the hosted version and the version previewed locally, you may be better suited reporting [seeking support in other forums](https://github.com/github/pages-gem/blob/master/docs/SUPPORT.md).*

### What did you do (e.g., steps to reproduce)
Removed the `jekyll-feed` plugin from my `_config.yml`

### What did you expect to happen?
Jekyll should stop generating feeds (with one post in my site, feed generation is take 4+ seconds on every "save".

```
Jekyll Feed: Generating feed for posts
...done in 4.326657 seconds.
```

### What happened instead?
GitHub Pages continues to inject `jekyll-feed` as a default plugin; `jekyll-feed` plugin injects `posts` as a required collection

### Additional Information

`jekyll-feed` is listed as an [optional plugin](https://help.github.com/en/articles/configuring-jekyll-plugins), but it does not appear to be optional at all.