rpc: remove scantxoutset EXPERIMENTAL warning
Summary:
Also remove an unnecessary leading "\n" (see D7184)
This is a backport of core#21426
Test Plan: src/bitcoin-cli help scantxoutset
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D11338