[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