[backport#15713] Tidy up BroadcastTransaction()
Summary:
https://github.com/bitcoin/bitcoin/pull/15713/commits/fb62f128bbfd8c6cd72ea8e23331a4bae23883ab
Concludes backport of Core PR15713
Test Plan:
ninja check-all
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D6427