Fix unused imports in lint/ python scripts

Summary: Preparation for a new linter on unused imports

Test Plan:

./test/lint/check-doc.py

No python error should occur

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2450