TASK: Remove legacy cr functional test artifacts

[mhsdesign]

• the sort operation test will be rewritten via !!! TASK: Remove leftovers from magic underscore access for internal node properties #5015 (comment)
• the content cache test will be rewritten via Fully migrate ContentCacheFlusherTest to behat #5037
• the TestObjectForSerialization has not been used or tested against in Neos 8.3 as well
• the other test things were not referenced and orphaned and thus removed

Upgrade instructions

Review instructions

Checklist

• Code follows the PSR-2 coding style
• Tests have been created, run and adjusted as needed
• The PR is created against the lowest maintained branch
• Reviewer - PR Title is brief but complete and starts with FEATURE|TASK|BUGFIX
• Reviewer - The first section explains the change briefly for change-logs
• Reviewer - Breaking Changes are marked with !!! and have upgrade-instructions

[nezaniel]

Do we already want to merge this as long as there are references like "xyz test will be rewritten in..." in here?
I'd like to first merge those new tests before removing old ones. Or are the old ones not being run anyway?

[mhsdesign]

Yes they are really not, but i put in issues what has to be migrated. Its just super annoying to always come across these in string replacements.

This pr mostly removes the site.xml things ... the real legacy tests ... the Neos ones are still there to be migrated with markAsSkipped.

So this removes just a super small portion that really annoyed me :D

[nezaniel]

Kill it with fire

[bwaidelich]

FYI: this one broke composer test script => #5095