[6.0] Removing Adapter and AdapterInstance classes

[Hackwar]

Summary of Changes

This PR removes the deprecated classes Adapter and AdapterInstance. It depends on #43792 and #43793. Those PRs remove the dependency on these classes. Thus this PR can only be merged when those 2 have been merged.

Testing Instructions

Codereview.

Link to documentations

Please select:

• [ ] Documentation link for docs.joomla.org:

• [X] No documentation changes for docs.joomla.org needed

• [X] Pull Request link for manual.joomla.org: https://github.com/joomla/Manual/pull/301

• [ ] No documentation changes for manual.joomla.org needed

[raksooo]

Thanks for the suggestion! We've added this to our backlog and will eventually look into it.

[securitybrahh]

What, this is a huge issue!

Should pin it.🤷