Fix EZP-20754: Not possible to delete a object containing ezimage datatype

This PR fixes issue https://jira.ez.no/browse/EZP-20754

The problem was in ```ImageStorage/Gateway/LegacyStorage::getXmlForImages()``` not loading correct set of fields to delete, and also in ```ImageStorage::deleteFieldData()```, where an exception was thrown if multiple fields (translations) shared same image.