[test] remove pointless block_hashes.append(...)

Summary: This line duplicated the block hashes, as they were already saved in that list using the list returned by generatetoaddress. It didn't cause any issue because all the following tests looped over the 101 first indices of that list, so the duplicated values were just ignored.

Test Plan:
with chronik
ninja check-functional

Reviewers: #bitcoin_abc, roqqit

Reviewed By: roqqit

Differential Revision: https://reviews.bitcoinabc.org/D16517