Move DeleteTimelineFlow::prepare to separate function and use enum

[arpad-m]

It was requested by review in #10305 to use an enum or something like it for distinguishing the different modes instead of two parameters, because two flags allow four combinations, and two of them don't really make sense/ aren't used.

follow-up of #10305

[github-actions]

7282 tests run: 6918 passed, 0 failed, 364 skipped (full report)

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Flaky tests (4)

Postgres 17

• test_subscriber_synchronous_commit: release-x86-64
• test_timeline_archival_chaos: release-arm64

Postgres 15

• test_physical_replication_config_mismatch_max_locks_per_transaction: release-arm64

Postgres 14

• test_pageserver_gc_compaction_smoke: release-x86-64

Code coverage* (full report)

• functions: 32.7% (8041 of 24618 functions)
• lines: 47.8% (66807 of 139904 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
686d769 at 2025-01-10T17:24:50.406Z :recycle:}