Remove "prepare" npm script that runs "dist"

[shawnbot]

This is a fix for one aspect of #715: if a repo/app/project uses a different version of Node than you've used to run npm install in this repo, running npm link will fail because it will attempt to build the CSS using a node-sass binary for the wrong version of Node.

So instead of running dist in our prepare script (which goes away), we run dist in the now-build script. I've tested this in dot-com and confirmed that bin/npm link ../../primer/css works after having run npm install in this repo with a different version of Node. ✨