pageserver: remove Option<> around remote storage, clean up metadata file refs

[jcsp]

Problem

This is historical baggage from when the pageserver could be run with local disk only: we had a bunch of places where we had to treat remote storage as optional.

Closes: #6890

Changes

• Remove Option<> around remote storage (in pageserver: refuse to run without remote storage #7722 we made remote storage clearly mandatory)
• Remove code for deleting old metadata files: they're all gone now.
• Remove other references to metadata files when loading directories, as none exist.

I checked last 14 days of logs for "found legacy metadata", there are no instances.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

3060 tests run: 2933 passed, 0 failed, 127 skipped (full report)

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Code coverage* (full report)

• functions: 31.4% (6338 of 20187 functions)
• lines: 47.4% (47915 of 101110 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
d79e9e5 at 2024-05-15T12:14:06.640Z :recycle:}

[VladLazar]

LGTM - did you already broadcast this on discord?

[VladLazar]

Also should your Closes link in the cover letter be https://github.com/neondatabase/neon/issues/6890 instead?

[koivunej]

approved with the thim typo fix pointed out by Vlad.