scripted-diff: use RPCArg::Optional::OMITTED over OMITTED_NAMED_ARG

-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-

src/rpc/util.h

@@ -165,7 +165,6 @@ struct RPCArg {
          * When possible, the default value should be specified.
          */
         OMITTED,
-        OMITTED_NAMED_ARG, // Deprecated alias for OMITTED, can be removed
     };
     /** Hint for default value */
     using DefaultHint = std::string;