Remove missing #bitcoinabc Freenode IRC channel from documentation
Summary: https://github.com/Bitcoin-ABC/bitcoin-abc/issues/114
Test Plan: NA
Reviewers: #bitcoin_abc, schancel, O1 Bitcoin ABC, deadalnix, Mengerian
Reviewed By: #bitcoin_abc, schancel, O1 Bitcoin ABC
Differential Revision: https://reviews.bitcoinabc.org/D845