wallet: Disallow abandon of conflicted txes

Summary:

Abandon transactions that are already conflicted is a noop, so don't try
and return false/throw instead.

Backport of core PR12282
https://github.com/bitcoin/bitcoin/pull/12282/files

Test Plan:

make check
./test/functional/test_runner.py wallet_abandonconflict

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3497