remove nonfunctional DSCP implementation

DSCP has not been fully working since the network manager was
introduced in 9.16, and has been completely broken since 9.18.
This seems to have caused very few difficulties for anyone,
so we have now marked it as obsolete and removed the
implementation.

To ensure that old config files don't fail, the code to parse
dscp key-value pairs is still present, but a warning is logged
that the feature is obsolete and should not be used. Nothing is
done with configured values, and there is no longer any
range checking.

lib/isc/netmgr/netmgr.c

@@ -123,13 +123,6 @@ static const isc_statscounter_t unixstatsindex[] = {
 };
 #endif /* if 0 */
 
-/*
- * Set by the -T dscp option on the command line. If set to a value
- * other than -1, we check to make sure DSCP values match it, and
- * assert if not. (Not currently in use.)
- */
-int isc_dscp_check_value = -1;
-
 static void
 nmsocket_maybe_destroy(isc_nmsocket_t *sock FLARG);
 static void