rpc: use RPCArg::Optional::OMITTED over OMITTED_NAMED_ARG

Summary:
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

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17943