Remove contrib/devtools/clang-format-diff.py

Summary:
Since clang-format is integrated into arcanist, this script became
useless.

Test Plan: Read contrib/devtools/README.md

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2574