My dumpster diving of binlogs showed that cases modtoken or multiPrimaryMode would be useful are so extremely rare that they can safely ignored (I didn't find one in half an hour of binlog). Hereby, I propose to simplify the code and our lives. First, let's set multiPrimaryMode to false in mainstash. Make sure everything is fine, then start cleaning up things.
Follow up to T383327: Re-architecture mainstash (x2) to allow easier maintenance
Effectively reverting https://gerrit.wikimedia.org/r/c/mediawiki/core/+/713925