rpc: Do not advertise dumptxoutset as a way to flush the chainstate

Summary:

The help message leaks several implementation details: leveldb and flush.

Neither of them are relevant to the end user and I don't see why we should make them part of the API contract.

This is a backport of Core PR18809

Test Plan: src/bitcoin-cli help dumptxoutset

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9067