Remove harmony-collections from build process.

This was added in https://github.com/atom/atom/commit/b9395d2946032a1a62a8b3d1627917ec854bd226
but doesn't appear to be necessary any longer as this build doesn't
use emissary during the build process.

This was causing a problem for me in my Atom Shell-based application
as I use grunt-typescript and grunt-tslint which die when WeakMap
has been provided by harmony-collections rather than the actual
Harmony support in V8.