Drop experimental timers

[mstoykov]

What?

Drop k6/experimental/timers

Why?

We have stabilized the API and it is now available for k6/timers for some time. We would like to stop supporting the old import path.

Checklist

• [ ] I have performed a self-review of my code.
• [ ] I have added tests for my changes.
• [ ] I have run linter locally (make lint) and all checks pass.
• [ ] I have run tests locally (make tests) and all tests pass.
• [ ] I have commented on my code, particularly in hard-to-understand areas.

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