[RFC] Simplified Homebrew formula

Removed unnecessary cmake modifications, and manual building of
dependencies. Updates to Makefile remove the need.
> So we should leave the wiki as it is to avoid confusion.

What about running `make (dist)clean` or `rm -rf ./build && make ...` (given in the [wiki](https://github.com/neovim/neovim/wiki/Installing#building) as an example to use another compiler)? Running CMake directly will clutter the neovim directory with CMake files and build Neovim twice (once in `build`, once in `src/nvim/...`) because the top-level `Makefile` is overwritten.