scansbr: Removed the ``performance hack'' for inc as it wasn't faster.

scansbr: Removed the ``performance hack'' for inc as it wasn't faster.
I tested a 150MB/500msgs mbox but couldn't see any time differences. In both
cases it took 2-5s on my machine. Hence, I go for the much simpler and not
``we use strange internals'' version. If you really inc much larger mboxes,
then just take some time. ;-)
The explaining comment for the hack had been:
    Performance hack: some people like to run "inc" on things like
    net.sources or large digests. We do a copy directly into the output
    buffer rather than going through an intermediate buffer.
        We need the amount of data m_getfld found & don't want to do a
    strlen on the long buffer so there's a hack in m_getfld to save the
    amount of data it returned in the global "msg_count".