Revert "[AUR] Add PKGBUILD files for the BCHN network variant"

Summary:
This reverts commit fb3ee22249cb86fd0e0ef3b9bcb4bc8985d6ca39.

Stop supporting the AUR repos for the BCHN variant.

Test Plan: Nothing to test here.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8682