Remove unused SetTip(nullptr) code

Summary:
Now that this path is no longer used after D13092, we can remove it.

Future code should reset CChain by simply discarding it and constructing a fresh one.

This is a backport of core#25023

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16069