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