[policy] Delete block policy estimator.
This commit removes the block policy estimator. It's currently basically useless,
and the algorithm for determining minimum fee isn't correct either. We will replace this
estimator using something that examines the active mempool.
make VERBOSE=1 check && ./test/functional/test_runner.py
Reviewers: #bitcoin_abc, schancel, markblundeberg
Reviewed By: markblundeberg
Subscribers: markblundeberg, teamcity
Differential Revision: https://reviews.bitcoinabc.org/D2172