This commit removes the logic that was added in #2045 to handle subscribers migrated from MP2 when deactivating subscribers. Without it, MP3 would deactive all subscribers imported from MP2 as the subscribe date is migrated but the stats are not (see https://mailpoet.atlassian.net/browse/MAILPOET-2040) for more details. This code is not necessary anymore as we are removing all the MP2 migration related code. [MAILPOET-4376]