undo SSH_BUG_RSASIGMD5 changes

If NERSC_MOD is activated, disable the SSH_BUG_RSASIGMD5 test as
described by:

 2013/12/30 23:52:28
     refuse RSA keys from old proprietary clients/servers that use the
     obsolete RSA+MD5 signature scheme. it will still be possible to
connect
     with these clients/servers but only DSA keys will be accepted, and
we'll
     deprecate them entirely in a future release. ok markus@