ADFA-4357 Drop backward-compat proposal and the now-empty Open-for-discussion section

The MIN_SDK guard concern doesn't arise in practice; remove the item. It was
the last proposal, so remove the empty section scaffolding too. REVIEW.md now
ends at §13 PR hygiene.

Author: hal-eisen-adfa

REVIEW.md

@@ -172,13 +172,3 @@ Features that aren't fully stabilized ship behind the **experimental feature fla
 - Focused and reviewable: aim for the **~500 LOC / 10-file** ceiling; split larger work into stacked PRs.
 - Title/branch follow `ADFA-####`; description says *what changed, why, how it was verified*, and flags anything intentionally out of scope (e.g. "UI-only, no unit test").
 - No stray debug logs, commented-out code, `dummy.apk`-style artifacts, or unrelated reformatting that buries the real diff.
-
----
-
-## Open for discussion (proposed additions)
-
-These aren't established rules yet — flagging them as candidates for the team:
-
-- **Backward compatibility:** `MIN_SDK=28` — review new APIs for guard/desugaring; remember user-built apps target `MIN_SDK_FOR_APPS_BUILT_WITH_COGO=16`.
-
-Add to or push back on any of these — this doc is meant to evolve with the team.