Drop support for OpenBSD

Summary:
Remove the associated documentation. OpenBSD instructions for building
bitcoin-abc with c++14 support are unknown. It can be re-added later if
someone needs it.

Test Plan: Read the remaining docs

Reviewers: #bitcoin_abc, deadalnix, jasonbcox, Mengerian

Reviewed By: #bitcoin_abc, jasonbcox, Mengerian

Subscribers: Mengerian, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2461