Removes unused dependencies from Gemfile
[Fixes #67428680] User can uncheck a data source on the data source index page.

 - A template helper method would clone the data source collection to build text for the sidebar, but _prepareModel of
   MultiModelSet does not handle cloning well. It appends the constructor name again to the id of the model. I.e. a model
   in the collection will get a unique id like 4GpdbDataSource, but when the collection is cloned, _prepareModel is called
   again and the id becomes 4GpdbDataSourceGpdbDataSource. This commit removes the clone of the collection from the template
   helper because it was unnecessary. Thus unchecking can properly remove the model from the selected models collection.
 - The issue still exists for cloning MultiModelSets