Remove dead code in BasicTestingSetup

Summary: This config is now unused. It was originally added to clear state between tests, but block size is no longer a global variable, so this code does nothing.

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3776