Remove alternative exports that have been deleted

[AntoineAugusti]

Currently, "alternative exports" on ODS are added as resources on udata

udata-ods/udata_ods/harvesters.py

Line 192 in 4a54c5c

self.process_extra_files(dataset, ods_dataset, 'alternative_export')

, but they are not removed when they have been removed on ODS.

This creates some problems:

• producers are confused
• some remote resources have a download link resulting in a 404
• reusers/partners see resources that have been deleted
• producers need to clean up resources on udata's backoffice

It would be awesome to "sync up" these resources and remove them when they do not exist anymore.