[9.18] 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files don't fail, the code to parse dscp key-value pairs is still present, but a warning is logged that the feature is obsolete and should not be used.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.

Backport of MR !7305 (merged)

Closes #3773 (closed)