METRO-61: MODS clean up self-transform to remove unused elements.

Impemented hook_islandora_derivative to apply a clean up self transform to the mods datastream.
Should probably be weighted above the [default DC stuff](https://github.com/Islandora/islandora_xml_forms/blob/7.x/builder/xml_form_builder.module#L738-L743) (default weight is assumed to be 0, go to -1)? Also, could we get `$derivatives` explicitly defined as an empty array at the top of the function? :P