Delete the contrib/verify-commits subtree

Summary:
This is a set of scripts to check that all commits are signed by someone
from a trusted list. This is not a feature we want, so drop the
directory to avoid maintenance cost to useless script.

Test Plan:

NOP();

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3955