refactor: Cleanup walletinitinterface.h

Summary:
Forward declarations of CScheduler and CRPCTable classes are no longer
needed after ea961c3d7256c66146b4976ab1293db4a628c0de commit.
Including <string> is no longer needed after
4d4185a4f0e40c033a587871839a47cb3f89ee93 commit.

This is a backport of Core PR16830

Thrown in chainparams.h for good measure.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6053