Filetranslation: Remove assocs to LanguageResources after translation

 Todo:

After a filetranslation in InstantTranslate is finished (=  right after the translation is done),

• remove the assocs to the LanguageResources that the task has used (remove the assocs only, not  the LanguageResources!),
• and log the removals in the task-log (so we can still check which LanguageResources have been used for the translation).

Benefit is:

• The tasks for filetranslations are hidden; when the assoc is removed, these tasks will no longer be visible in the LanguageResources' list of associated tasks.
• LanguageResources can be removed even if the hidden task for a filetranslation still exists and can be exported.

Update IP 2020-04-15: Assocs are removed also after import stops with an error.