Fix unused imports in abc-* functional tests

Summary: Preparation for a new linter that will check for these unused imports.

Test Plan:

./test/functional/test_runner.py abc-cmdline abc-finalize-block

abc-high_priority_transaction abc-magnetic-anomaly-mining
abc-mempool-accept-txn abc-p2p-compactblocks abc-p2p-fullblocktest
abc-parkedchain abc-rpc abc-sync-chain abc-transaction-ordering

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2446