Shapefile datastore support for removeSchema

The missing implementation of removeSchema prevents GeoServer to make a proper cleanup of an ImageMosaic coverage data store (using the purge=all parameter on the REST call for removing a coverage store) in the case where a shapefile is used as datastore for indexing the image mosaic. Especially the .properties file is not removed, which prevents re-uploading the ImageMosiac data store to the GeoServer using the same same storage name and the same image data.