refactor(localdb): delete the dormant LiteDB LocalCache (superseded by ZB.MOM.WW.LocalDb)

Never registered in DI and dead since Phase 6.1. Keeping two unwired cache designs
invites the next reader to wire the wrong one.

Deletes all three test files - LiteDbConfigCacheTests.cs was missing from the plan's
list and would have failed to compile (it calls new LiteDB.LiteDatabase directly).
StaleConfigFlagTests lives inside ResilientConfigReaderTests.cs, so it goes too.

The XML-doc reference in ILdapGroupRoleMappingService is rewritten rather than removed:
it now records that no sign-in fallback exists and that reviving one means an admin-side
cache on LocalDb.

docs/plans/2026-07-20-localdb-phase1-recon.md

@@ -469,6 +469,20 @@ Plan `…phase1.md:484-485` names only `GenerationSealedCacheTests.cs` and
 
 See D-3.
 
+### 7.3b Outcome (Task 9, executed 2026-07-20)
+
+Deleted: the 6 `LocalCache/` sources and **all three** test files (including
+`LiteDbConfigCacheTests.cs`, which the plan omitted — D-3). `LiteDB` removed from both
+`Configuration.csproj` and `Directory.Packages.props`. The stale XML-doc reference at
+`ILdapGroupRoleMappingService.cs:14` was rewritten to state the present truth (no fallback exists;
+reviving it means an admin-side cache on LocalDb, not restoring the old pipeline) rather than
+deleted, per the DoD's "explanatory prose may remain". Configuration tests: 92/92 green.
+
+**Follow-up found while executing:** `Polly.Core` in `Configuration.csproj` is now orphaned too —
+`ResilientConfigReader` was its only real consumer in that project (remaining `Polly` mentions are
+comments). Left in place deliberately: removing it is outside Task 9's scope and could break a
+consumer relying on the transitive flow. Worth a separate cleanup.
+
 ### 7.4 Package removal
 
 - `PackageReference`: `Configuration.csproj:29` — only consumer.