pageserver: remove absolute-order disk usage eviction

[jcsp]

Problem

Deployed pageserver configurations are all like this:

      disk_usage_based_eviction:
        max_usage_pct: 85
        min_avail_bytes: 0
        period: "10s"
        eviction_order:
          type: "RelativeAccessed"
          args:
            highest_layer_count_loses_first: true

But we're maintaining this optional absolute order eviction, with test cases etc.

Summary of changes

• Remove absolute order eviction. Make the default eviction policy the same as how we really deploy pageservers.

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]

3114 tests run: 2993 passed, 0 failed, 121 skipped (full report)

---

Flaky tests (1)

Postgres 16

• test_scrubber_physical_gc_ancestors[2]: release

Code coverage* (full report)

• functions: 32.6% (7000 of 21461 functions)
• lines: 50.0% (55274 of 110514 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
2f14bb8 at 2024-07-22T11:28:48.974Z :recycle:}