Remove blockFinished from BlockAssembler

Summary: As per title, stop using spooky action at a distance

Test Plan:

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

Reviewers: #bitcoin_abc, jasonbcox, deadalnix

Reviewed By: #bitcoin_abc, jasonbcox, deadalnix

Subscribers: deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1591