MAINT: Remove tools/osxbuild

These scripts have been broken for years (build.py errors out
immediately if not running from an *svn* checkout), so I'm sure
no-one's using them, and they only attract pointless maintenance fixes
(py3 changes, keeping the pointless extra copy of the top-level
README.txt up to date as in #3288). Let's just remove them.
Merge pull request #3349 from njsmith/remove-tools-osxbuild

MAINT: Remove tools/osxbuild