It's unnecessary overhead for anything which does Email::MIME
parsing. It was never done for v2 indexing, even though v1->v2
conversions did NOT remove those From_ lines. There was never a
need to remote From_ lines the v1 SearchIdx paths, either.
Hitting a /$INBOX_URL/$MSGID/T/ endpoint with an 18 message
thread reveals a ~0.5% speed improvement. This will become
more apparent when we have a faster MIME parser.