docs(arc): remove metrics which are not exposed

[raffis]

Why:

Current arc docs lists two metrics which are not exposed.
In the pr which added these metrics available_jobs and acquired_jobs are actually commented:
actions/actions-runner-controller@a0a3916#diff-45afc66b23a4ce69ef91a1ee70fd48b378fa89ad649c7c2275ccff5516f9e497R54-R70

What's being changed (if available, include any code snippets, screenshots, or gifs):

Remove metrics from the docs (until the metrics are actually exposed by the controller),

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Source	Preview	Production	What Changed
actions/hosting-your-own-runners/managing-self-hosted-runners-with-actions-runner-controller/deploying-runner-scale-sets-with-actions-runner-controller.md	fpt ghec ghes@ 3.10 3.9	fpt ghec ghes@ 3.10 3.9

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[cmwilson21]

@raffis Thanks so much for submitting a PR! I'll get this triaged for review ⚡

[felicitymay]

Many thanks for linking to the PR that introduces these two new metrics and highlighting the fact that they have not shipped yet. ✨

Looking at master I can see that they are still commented out.

Thank you for letting us know about this error and for fixing it 🚀

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