Remove the WP Slug Trimmer plugin

[matt-bernhardt]

Why are these changes being introduced:

• The WP Slug Trimmer plugin, one of our abandoned plugins which is deployed via a private repository, has a bug that prevents it from functioning on PHP 8. In digging a bit deeper in the legacy site, it may not be functioning there either (although it also does not throw errors and break the site like it does under PHP 8).

  Our choices, as laid out in LM-287, are to either take ownership over the plugin and fix the bug (and then investigate whether it actually works), or to retire the plugin entirely.

Relevant ticket(s):

• https://mitlibraries.atlassian.net/browse/LM-287

How does this address that need:

• After discussing the functionality provided by this plugin with Darcy, she made the decision to just retire the plugin rather than try to bring it up to functionality.
• This PR removes that plugin from the network.

How to confirm this change

• Log into the Live tier on Pantheon, go to the News dashboard, and try to publish a Post (just a title and body field are needed, not all the other fields). Note that the application crashes.

• Additionally, try to load the Settings page for Slug Trimmer (Settings -> Slug Trimmer). Note that this page also crashes.

• Log into the review app for this PR. Note that Slug Trimmer is no longer on the list of available plugins. Publish a Post (same restrictions, just a title and body field). Note that the Post is created successfully.

Document any side effects to this change:

• None

Developer

Secrets

• No secrets are affected (although there's now one less use for the Github OAuth token)

Documentation

• Project documentation has been updated - plugin is noted as being removed on the Plug-In Documentation

Accessibility

• ANDI or Wave has been run in accordance to
  our guide and
  all issues introduced by these changes have been resolved or opened as new
  issues (link to those issues in the Pull Request details above)

Stakeholder approval

• Stakeholder approval has been confirmed

Dependencies

YES dependencies are updated

Code Reviewer

• The commit message is clear and follows our guidelines
  (not just this pull request message)
• The changes have been verified
• The documentation has been updated or is unnecessary
• New dependencies are appropriate or there were no changes

[JPrevost]

Can you remove it from https://github.com/MITLibraries/mitlib-wp-network/blob/master/docs/howto/test-plugins.md#slug-trimmer as well?

[matt-bernhardt]

Ooh, actually I think I need to remove that entire page. When Darcy started reviewing that document, she made a copy in Google Docs, and we haven't decided yet where the page will live officially. Until that determination is made, I've been updating the Google Doc, and not making any changes to this markdown file.

I'll push a commit to remove that markdown file.