Remove a source from the state.
@param {Object} state Current state.
@param {Object} action Action to handle.

@returns {Object} The new state.
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));
}