Remove prepack script.

This was originally added to resolve #50, but I realized `npm publish` runs both prepublishOnly *and* prepack, meaning we'd be building twice.

Author: Phanabani

package.json

@@ -11,7 +11,6 @@
   },
   "scripts": {
     "build": "parcel build",
-    "prepack": "yarn run build",
     "prepublishOnly": "yarn run build",
     "test": "echo \"Error: no test specified\" && exit 1",
     "watch": "parcel watch",