Remove communication style article from CONTRIBUTING.md

Summary:
Although the article may be useful as general advice, it seems out of place with the other articles.
It's better to keep these articles focused on specific development-style information that people may not easily find elsewhere.

Test Plan: N/A

Reviewers: deadalnix, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: schancel

Differential Revision: https://reviews.bitcoinabc.org/D2781