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/Noise/NoiseSession.swift

@@ -296,19 +296,6 @@ class NoiseSessionManager {
         }
     }
     
-    func migrateSession(from oldPeerID: String, to newPeerID: String) {
-        managerQueue.sync(flags: .barrier) {
-            // Check if we have a session for the old peer ID
-            if let session = sessions[oldPeerID] {
-                // Move the session to the new peer ID
-                sessions[newPeerID] = session
-                _ = sessions.removeValue(forKey: oldPeerID)
-                
-                SecureLogger.log("Migrated Noise session from \(oldPeerID) to \(newPeerID)", category: SecureLogger.noise, level: .info)
-            }
-        }
-    }
-    
     func getEstablishedSessions() -> [String: NoiseSession] {
         return managerQueue.sync {
             return sessions.filter { $0.value.isEstablished() }