Remove the tested versions from the OSX build guide

Summary:
Unless someone effectively tests these versions for each update, I don't
think it's worth keeping it into the documentation. This might lead the
users to expect more than what we can guarantee.
Note: I use a single 10.12.6 version for my OSX tests.

Test Plan: Read it.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3907