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]
