page cache metrics: remove unused read_accesses_ephemeral & read_hits_ephemeral

[problame]

We removed the user of this in #4994 .

But the metrics field was pub, so, didn't cause an unused-warning in #4994.

This is preliminary for: #5339

[github-actions]

2496 tests run: 2379 passed, 0 failed, 117 skipped (full report)

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Flaky tests (2)

Postgres 16

• test_remote_storage_backup_and_restore[True-local_fs]: debug

Postgres 14

• test_crafted_wal_end[last_wal_record_xlog_switch_ends_on_page_boundary]: debug

Code coverage (full report)

• functions: 53.0% (7787 of 14702 functions)
• lines: 81.1% (45419 of 56007 lines)

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_{The comment gets automatically updated with the latest test results
fa64e7e at 2023-09-20T16:05:54.495Z :recycle:}