Delete README_osx.md and move its contents into build-osx.md
Summary:
With its current name, the file doc/README_osx.md looks like an entry point README for OSX users, but it only contains specific instructions on how to build a DMG. This PR deletes the file and moves the contents of the file into doc/build-osx.md.
Backport of core PR15176.
Test Plan: Read the doc.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D5512