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/ViewModels/ChatViewModel.swift

@@ -1474,10 +1474,7 @@ class ChatViewModel: ObservableObject, BitchatDelegate {
         privateChats.removeAll()
         unreadPrivateMessages.removeAll()
         
-        // First run aggressive cleanup to get rid of all legacy items
-        _ = KeychainManager.shared.aggressiveCleanupLegacyItems()
-        
-        // Then delete all current keychain data
+        // Delete all keychain data
         _ = KeychainManager.shared.deleteAllKeychainData()
         
         // Clear UserDefaults identity fallbacks