[policy] Delete block policy estimator.

Summary:
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.

Depends on D2037
Complete T499

Test Plan:

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