Remove BTCForks Slack from CONTRIBUTING.md
Summary: It is pretty much unused, so it's confusing to send new people there.
Test Plan: N/A
Reviewers: Fabien, jasonbcox, deadalnix, #bitcoin_abc
Reviewed By: deadalnix, #bitcoin_abc
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2182