Do not delete nc pvc when deleting helm chart

when you purge nextcloud, the mariadb pvc stays, but the nextcloud pvc gets deleted. This causes a mismatch when you re-install nextcloud.

Our current solution is to always remove the mariadb pvc, so you get a fresh and new nextcloud every time you reinstall, but maybe, if we keep the nextcloud pvc, you can completely restore your nextcloud from those two PVCs after you've deleted it!