matrix: Remove retention policy backfill code

[backspace]

No description provided.

[github-actions]

Host Test Results

    1 files  ±0      1 suites  ±0   21m 51s ⏱️ +6s
734 tests ±0  732 ✔️ ±0  2 💤 ±0  0 ❌ ±0 
738 runs  ±0  736 ✔️ ±0  2 💤 ±0  0 ❌ ±0 

Results for commit 752d244. ± Comparison against base commit 0fa1e68.

This pull request removes 341 and adds 341 tests. Note that renamed tests count towards both.

Chrome 131.0 ‑ Acceptance | code submode | file-tree tests > when the user lacks write permissions: it is reflected in the realm header
Chrome 131.0 ‑ Acceptance | code submode | file-tree tests: can navigate file tree, file view mode is persisted in query parameter
Chrome 131.0 ‑ Acceptance | code submode | file-tree tests: can open files
Chrome 131.0 ‑ Acceptance | code submode | file-tree tests: can open files in base realm
Chrome 131.0 ‑ Acceptance | code submode | file-tree tests: can scroll to bottom of the file tree
Chrome 131.0 ‑ Acceptance | code submode | file-tree tests: navigating to a file in a different realm causes it to become active in the file tree
Chrome 131.0 ‑ Acceptance | code submode | file-tree tests: open directories are persisted
Chrome 131.0 ‑ Acceptance | code submode | file-tree tests: open file is within view even when its parent directory is not stored as open
Chrome 131.0 ‑ Acceptance | code submode | file-tree tests: open file is within view when the file browser renders
Chrome 131.0 ‑ Acceptance | code submode | file-tree tests: opening another file preserves the scroll position
…

Chrome 131.0 ‑ Acceptance | code submode tests > multiple realms: default realm is the personal realm
Chrome 131.0 ‑ Acceptance | code submode tests > multiple realms: first item in add file realm dropdown is the currently displayed realm
Chrome 131.0 ‑ Acceptance | code submode tests > single realm > with connection to test realm: code submode handles binary files
Chrome 131.0 ‑ Acceptance | code submode tests > single realm: Clicking card in search panel opens card JSON in editor
Chrome 131.0 ‑ Acceptance | code submode tests > single realm: can handle error when user puts unidentified domain in card URL bar
Chrome 131.0 ‑ Acceptance | code submode tests > single realm: card preview live updates when index changes
Chrome 131.0 ‑ Acceptance | code submode tests > single realm: card preview live updates with error
Chrome 131.0 ‑ Acceptance | code submode tests > single realm: card preview will show in the 3rd column when submode is set to code
Chrome 131.0 ‑ Acceptance | code submode tests > single realm: card-catalog does not offer to "create new card" when editing linked fields in code mode
Chrome 131.0 ‑ Acceptance | code submode tests > single realm: changes cursor position when selected module declaration is changed
…