reduce scope of mbox From_ line removal

reduce scope of mbox From_ line removal

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.