[website] remove bitcoincash.org roadmap link

Summary: We don't need to link to bitcoincash.org anymore

Test Plan: Built it locally, looks OK

Reviewers: Fabien, deadalnix, johnkuney, #bitcoin_abc, josephroyking

Reviewed By: #bitcoin_abc, josephroyking

Subscribers: josephroyking

Differential Revision: https://reviews.bitcoinabc.org/D8688