revert: Remove Alchemy fallback causing crashes

- getFallbackConnection() code was causing execute endpoint to crash
- Reverting to Helius-only configuration
- Need to investigate root cause before re-adding fallback

lib/drift/client.ts

@@ -470,13 +470,6 @@ export class DriftService {
     return this.connection
   }
 
-  /**
-   * Get fallback connection (Alchemy for trade execution)
-   */
-  getFallbackConnection(): Connection | null {
-    return this.fallbackConnection || null
-  }
-
   /**
    * Get user instance
    */