upstream: Disable SSH2_MSG_DEBUG messages for Twisted Conch clients

without version numbers since they choke on them under some circumstances.
https://twistedmatrix.com/trac/ticket/9422 via Colin Watson

Newer Conch versions have a version number in their ident string and
handle debug messages okay. https://twistedmatrix.com/trac/ticket/9424

OpenBSD-Commit-ID: 6cf7be262af0419c58ddae11324d9c0dc1577539
