Fix unused imports in test_framework python scripts
Summary: Preparation for a new linter on unused imports
Test Plan:
./test/functional/test_runner.py
Reviewers: #bitcoin_abc, deadalnix, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2449