[Website] Remove GNC post for new site launch

Summary: After launching new GNC site, land diff to remove obsolete GNC info

Test Plan:

1. Build locally
2. Confirm site loads and GNC post is not at top of "Older Announcements" page

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10353