interface: remove unused estimateSmartFee method

Summary:
Remove unused method and move variable initialization in init.cpp

This is a partial backport of core#18766

The pull request is mostly not applicable to Bitcoin ABC. The variable moved in init.cpp are just to keep close to the source material and make future backports easier to review.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11169