osx has been extracted to its own repo

- [#70019034]
- https://github.com/pivotal-sprout/osx
Use cask LWRP from homebrew cookbook
- effectively deprecates our custom LWRP
- homebrew::base recipe is pretty fiddly and does appear to converge reliably
Remove osx cookbook from Rakefile
Update Cheffile to reflect recent extraction of osx cookbook
Remove charles_proxy recipe

It had a weird dependency into pivotal_workstation and using a cask is better
Remove sprout-pivotal and pivotal_workstation

These have been moved to sprout-attic
CONVENTIONS.md is superceded by sprout-exemplar
Update/remove legacy files

- sprout-osx-apps will instead follow sprout-exemplar conventions
- it will soon be the top level folder in this repo
Make sprout-osxapps/ the top-level folder

Since there is only 1 cookbook remaining in this repo