Remove unused param from txmempool::estimateFee

Summary:
Cleanup to help simplify the coincontrol logic being backported in T521.
Depends on D2613, D2641, D2662
Split out from D2543 for better review

Test Plan:
ninja check
test_runner.py rpc_estimatefee

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2614