Remove prepack script from each plugin

Summary: Removed back "prepack" script from every plugin package to keep it simple. Instead we will use `build-plugin` command defined in the root package.json (see next diff in the stack).

Reviewed By: mweststrate

Differential Revision: D22160252

fbshipit-source-id: cffb5df65b89df3fe390c051da0df797b3d16d2d

desktop/plugins/sections/package.json

@@ -20,8 +20,5 @@
   },
   "resolutions": {
     "react-d3-tree/d3": "file:./d3"
-  },
-  "scripts": {
-    "prepack": "../../pkg/bin/run bundle"
   }
 }