Revert "Test with JIT compiler on latest Ruby"

[bquorning]

This reverts commit b00329e / PR #6456.

I don't think we have seen any value from running tests with the JIT compiler enabled – so let’s just disable it again.

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Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[koic]

I agree.
AFAIK, MJIT may be disabled by default in Ruby 3.0. This is because memory usage increases instead of gaining runtime speed. For example, there is a risk that it won't work with Heroku's smallest dyno.
Of course, I don't know yet what will happen in the future :-)