Cleanup useless dependency in setup-debian-buster.sh

Summary:
apt-cacher-ng is not used within our builds, though APT caching may be
available on the build agent, this is transparent to the build itself.

Test Plan: Run some tests on CI

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6057