chore(compute): Do a minor cleanup of remote extensions code

[ololobus]

Problem

TBD

Summary of changes

Started within a different PR, but then realized that I made too many changes, so moved to a separate one.

[github-actions]

7403 tests run: 7017 passed, 0 failed, 386 skipped (full report)

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Flaky tests (10)

Postgres 17

• test_scrubber_tenant_snapshot[4]: debug-x86-64-without-lfc
• test_metrics_normal_work: release-x86-64-with-lfc, release-x86-64-without-lfc, release-arm64-with-lfc
• test_timeline_size_createdropdb: debug-x86-64-without-lfc

Postgres 16

• test_metrics_normal_work: release-arm64-with-lfc

Postgres 15

• test_sql_regress[4]: release-arm64-with-lfc
• test_pg_regress[4]: release-arm64-with-lfc
• test_metrics_normal_work: release-x86-64-with-lfc

Postgres 14

• test_metrics_normal_work: release-arm64-with-lfc

Code coverage* (full report)

• functions: 33.5% (8498 of 25347 functions)
• lines: 49.3% (71493 of 145082 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
8a1ed6a at 2025-01-27T19:29:31.182Z :recycle:}