refactor: remove all dead legacy and migration code

- Remove unused migrateSession() functions (never called in production)
  - NoiseSession.migrateSession() - 13 lines
  - NoiseEncryptionService.migratePeerSession() - 18 lines
  - Test for migration functionality - 17 lines

- Remove unnecessary keychain cleanup code (no legacy data existed)
  - cleanupLegacyKeychainItems() - 62 lines
  - aggressiveCleanupLegacyItems() - 72 lines
  - resetCleanupFlag() - 4 lines
  - Simplified panic mode to just use deleteAllKeychainData()

Total removed: 194 lines of dead/unnecessary code

Analysis revealed:
- KeychainManager introduced July 5, 2025
- Cleanup code added July 15, 2025 (10 days later)
- Legacy service names were never used in production
- Migration functions were incomplete implementation never called
- Peer ID rotation remains active (not legacy)

bitchat/Services/NoiseEncryptionService.swift

@@ -419,24 +419,6 @@ class NoiseEncryptionService {
         SecureLogger.logSecurityEvent(.sessionExpired(peerID: peerID))
     }
     
-    /// Migrate session when peer ID changes
-    func migratePeerSession(from oldPeerID: String, to newPeerID: String, fingerprint: String) {
-        // First update the fingerprint mappings
-        serviceQueue.sync(flags: .barrier) {
-            // Remove old mapping
-            if let oldFingerprint = peerFingerprints[oldPeerID], oldFingerprint == fingerprint {
-                peerFingerprints.removeValue(forKey: oldPeerID)
-            }
-            
-            // Add new mapping
-            peerFingerprints[newPeerID] = fingerprint
-            fingerprintToPeerID[fingerprint] = newPeerID
-        }
-        
-        // Migrate the session in session manager
-        sessionManager.migrateSession(from: oldPeerID, to: newPeerID)
-    }
-    
     // MARK: - Private Helpers
     
     private func handleSessionEstablished(peerID: String, remoteStaticKey: Curve25519.KeyAgreement.PublicKey) {