Remove confusing statement from setban RPC help text

Summary:
This statement is not technically true, as restarting the node
will revert automatic/discouraging bans.

This brings our code more inline with Core.

Test Plan:

ninja
bitcoind
bitcoin-cli help setban    # verify new help text

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6865