Get rid of "use X11 if it's installed" deps

I want to get rid of these because:

 * they are more confusing than a plain optional/recommend dep
 * they can't be tracked reliably because of the conditional
 * they present a roadblock to bottling some things
 * the first time `MacOS::X11.installed?` is called, we shell out to `mdfind` and `mdls`, which makes formula loading slower than it needs to be