[Clean-up] Remove unnecessary default: NOTREACHED() case.

The (clang) compiler is smart enough to catch this error at compile-time, which
is better than waiting until runtime.

BUG=none
TEST=none
R=keybuk@chromium.org

Review URL: https://codereview.chromium.org/575343002

Cr-Commit-Position: refs/heads/master@{#297060}

extensions/browser/api/bluetooth_socket/bluetooth_api_socket.cc

@@ -140,8 +140,6 @@ void BluetoothApiSocket::OnSocketReceiveError(
     case device::BluetoothSocket::kSystemError:
       error_reason = BluetoothApiSocket::kSystemError;
       break;
-    default:
-      NOTREACHED();
   }
   error_callback.Run(error_reason, message);
 }