[website] Remove Bitcoin Cash from project description

Summary: We don't support BCH anymore, so remove it from the description of Bitcoin ABC

Test Plan: Built and ran locally. check to make sure it looks OK and that the message reads well and conveys what we want it to.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8996