[rpc] remove estimatesmartfee api

Summary:
Remove estimatesmartfee API. It isn't particularly smart.
estimatefee is a suitable replacement for the time being.

Test Plan:

make check && ./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1363