Cleanup unused proofCount

Summary:
Early exit by tracking proofCount became unworkable in D14663

It is now no longer used.

Test Plan:

ninja check && ninja check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16410