refactor(localdb): delete the dormant LiteDB LocalCache (superseded by ZB.MOM.WW.LocalDb)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

src/Core/ZB.MOM.WW.OtOpcUa.Configuration/Services/ILdapGroupRoleMappingService.cs

@@ -10,10 +10,18 @@ namespace ZB.MOM.WW.OtOpcUa.Configuration.Services;
 ///     Phase 6.2 compliance check on control/data-plane separation).
 /// </summary>
 /// <remarks>
-///     Per Phase 6.2 Stream A.2 this service is expected to run behind the Phase 6.1
-///     <c>ResilientConfigReader</c> pipeline (timeout → retry → fallback-to-cache) so a
-///     transient DB outage during sign-in falls back to the sealed snapshot rather than
-///     denying every login.
+///     <para>
+///         This service has no local-cache fallback: a DB outage during sign-in denies logins.
+///     </para>
+///     <para>
+///         The Phase 6.1 <c>ResilientConfigReader</c> pipeline (timeout → retry →
+///         fallback-to-sealed-snapshot) this was once expected to run behind was never wired to
+///         anything and was deleted along with the rest of the dormant LiteDB local cache, which
+///         <c>ZB.MOM.WW.LocalDb</c> supersedes. LocalDb caches the deployed-configuration artifact
+///         for driver-role nodes; it does not currently cover admin-plane reads like this one.
+///         Reviving the fallback means adding an admin-side cache on LocalDb, not restoring the
+///         old pipeline.
+///     </para>
 /// </remarks>
 public interface ILdapGroupRoleMappingService
 {