Remove experimental parallel feature

[MichaReiser]

Remove the experimental parallel feature.

We still want something like this in the future, but the tests are flaky (Miri now gets stuck consistently)
and neither ty or r-a are using the feature (and there's no other maintainer driving the feature forward).

The feature is also surprising, because you aren't supposed to use it within a query because
The outer query won't track the dependencies of the queries running in parallel (which raises questions about how this would work with maybe_changed_after).

Parallel execution with Salsa is possible even without this feature. You can clone your db (add a dyn_clone method to your Db trait if you only have a &dyn Db)
and pass a separate instance to each thread.

Closes #994

[netlify]

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[MichaReiser]

We still need the rayon feature for the salsa::Update implementation for rayon::Either

[codspeed-hq]

CodSpeed Performance Report

Merging #1013 will improve performances by 4.37%

_{Comparing MichaReiser:remove-parallel (b0787c2) with master (a4113cd)}

Summary

⚡ 1 improvement
✅ 12 untouched

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	amortized[SupertypeInput]	2.8 µs	2.7 µs	+4.37%