code: @MainThread
      public <S> void removeSource(@NonNull LiveData<S> toRemote) {
          Source<?> source = mSources.remove(toRemote);
          if (source != null) {
              source.unplug();
          }
      }
docs: Stops to listen the given {@code LiveData}.
  
  @param <S>      the type of data hold by {@code source} LiveData
  @param toRemote {@code LiveData} to stop to listen