revert(db): drop unused migration policy seed

The explicit-only gating design it anchored was superseded by the
directive to remove branch-added migration machinery entirely. Surgical
two-file removal; the background_reads addition from the same commit
window stays.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: ScriptedAlchemy

crates/tracedecay-runtime-core/src/db/migration_policy.rs

@@ -13,7 +13,6 @@ mod maintenance;
 mod memory_connection;
 mod memory_v2;
 mod metadata;
-pub mod migration_policy;
 pub mod migrations;
 mod nodes;
 #[cfg(test)]