Revert "[GITIAN] Build for both chains"

Summary:
This reverts commit 0e442bc8bbee7251cbeb5c1dd2fa7cf402ad0e5d.

Stop distributing the BCHN variant for Bitcoin ABC.

Test Plan: Run the gitian builds.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8680