clear logs

Summary:
- clear logs using executeShell
- remove unused spawnShell

Reviewed By: jknoxville

Differential Revision: D17343235

fbshipit-source-id: 4353b2ebc00e35b31e815e405534a78b65a3eabb

src/devices/BaseDevice.tsx

@@ -135,10 +135,6 @@ export default class BaseDevice {
     this.logListeners.delete(id);
   }
 
-  spawnShell(): DeviceShell | null | undefined {
-    throw new Error('unimplemented');
-  }
-
   navigateToLocation(location: string) {
     throw new Error('unimplemented');
   }