refactor: Remove unused methods CBloomFilter::reset()/clear()

Summary:
Co-authored-by: MarcoFalke <falke.marco@gmail.com>

The method CBloomFilter::reset() was introduced by commit d2d7ee0 in 2015, but was never ever used, as far as I could find. As discovered by MarcoFalke, the method clear() is also unused outside of unit tests and is hence also removed.

This is a backport of Core PR18670

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8937