code: function removeSource(state, action) {
    const new_sources = Object.assign({}, state.sources);
    delete new_sources[action.sourceName];
    return Object.assign({}, state, {
      sources: new_sources,
    }, incrementVersion(state.metadata, SOURCE_VERSION_KEY));
  }
docs: Remove a source from the state.
  @param {Object} state Current state.
  @param {Object} action Action to handle.
  
  @returns {Object} The new state.