Remove txmempool::estimateSmartFee as it's redundant with ::estimateFee

Summary:
This will help simplify backporting and the review of future backports now that there is no
estimateSmartFee. estimateSmartFee's params are useless and we shouldn't be encouraging backports
to keep evaluation of those param values around.

Test Plan:

make check
test_runner.py

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2540