Feat/keep stats but remove user

[ajeanneney]

Issue description :

Delete only treatmentSumary and not all statistics on purge

Describe your changes :

Unset treatmentsumary instead of deleting statistic.

How to test :

insert statistics and run purgeDb script
Waring : if the user mentioned in the treatment sumary is not found in database the search for specific statistic failed

Checklist before requesting a review

• I have performed a self-review of my code.
• The feature works locally.
• If it's relevant I added tests.
• Will this be part of a product update? If yes, please write one phrase about this update.

[Bouba-cassation]

J'ai lancé la branche en local et tester.

• Tout est bon, quand on supprime un treatmentsSummary (set : []), les stats sont toujours affichées, par contre nous n'avons plus le traimentment duration du document.

[Bouba-cassation]

J'ai testé en local en :

• Updatant le treatmentDate puis lancer le script purgeDb.js dans l'intervale (--beforeMonths 1). Ca supprime l'utilisateur et laisse le treatmentDuration.