Remove legacy announcements feed

Summary:
Our most recent announcements have been ported over to blog.bitcoinabc.org,
making this feed section redundant with the one above it. Time to clean it up.

Test Plan: make serve

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7313