Remove deprecated RPCArg::Optional::OMITTED_NAMED_ARG in favour of OMITTED.
See D16315 for the deprecation.
-BEGIN VERIFY SCRIPT- sed -i -e "/Deprecated alias for OMITTED, can be removed/d" src/rpc/util.h src/rpc/util.cpp sed -i -e "s/OMITTED_NAMED_ARG/OMITTED/g" $(git grep -l "OMITTED_NAMED_ARG" src/) -END VERIFY SCRIPT-
Also improve doc for RPCArg::Optional::OMITTED.
This is a backport of core#26919
Depends on D17942