[RFC] Simplified Homebrew formula

Removed unnecessary cmake modifications, and manual building of
dependencies. Updates to Makefile remove the need.
If you remove the `deps`, Neovim will be built twice (once here, once below at `make install`). You could also do it with just two commands:

```bash
make CMAKE_EXTRA_FLAGS="-DCMAKE_PREFIX_PATH=#{prefix}"
make install
```