Fix unused imports in rpc_* functional tests

Summary: Preparation for a new linter on unused imports

Test Plan:

./test/functional/test_runner.py rpc_fundrawtransaction

rpc_listtransaction rpc_named_arguments rpc_signmessage

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2448