Fix unused imports in feature_*, mempool_* and p2p_* functional tests

Summary: Preparation for a new linter on unused imports

Test Plan:

./test/functional/test_runner.py feature_bip68_sequence

feature_minchainwork mempool_limit p2p_fingerprint

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2447