Thanks to T374544: Use "control" as the static default for growthexperiments-homepage-variant, we can now (once wmf.26 is branched, ie. starting October 07) drop rows from user_properties table at Wikimedia wikis. Namely, we can drop all rows with up_property=growthexperiments-homepage-variant, besides those where the value is oldimpact.
This can be done via userOptions.php (--delete-defaults and/or --delete --old=VALUE-TO-DELETE should be the parameters to use).