Revert "Build the debian package for both chains"

Summary:
This reverts commit bb66fadd7acdd0e8cb198a9d406b823fee38da8d.

Stop supporting the BCHN variant PPA.

Test Plan:

./contrib/release/debian-packages.sh --dry-run fabien

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8681