Remove inout from JSON valuetype getters

[Erikvv]

After feedback of #6682

[dlang-bot]

Thanks for your pull request and interest in making D better, @Erikvv! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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• My PR is fully covered with tests (you can see the annotated coverage diff directly on GitHub with CodeCov's browser extension
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Testing this PR locally

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dub fetch digger
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[n8sh]

You've unintentionally included #6682. Other than that, thumbs up.

[n8sh]

Removed the extra commit, and approved.