Remove unused DoWarning function

Summary:
Since BIP9 code has been removed in D1539 and now D3181 removed the other
usage, this static function is now unused and generates compiler warning.

Depends on D3181.

Test Plan: compile bitcoind

Reviewers: nakihito, jasonbcox, #bitcoin_abc, deadalnix, Fabien

Reviewed By: nakihito, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3231