Clarify flow of Mac install docs

This removes a highly confusing paragraph about installing Docker with
Homebrew (a completely different install path) in the middle of the
main install path.

The Homebrew install path is removed entirely. The rationale is that
1) the mac install is complicated enough without confusing people with
an alternative, and 2) people who already have homebrew will simply
'brew search' and shouldn't need this doc in the first place.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)