Don't set ASSET_HOST on build:development (#3936)

Setting ASSET_HOST to `http://0.0.0.0:8080` makes urls in manifest.json to
be invalid, e.g. `http://0.0.0.0:8080/packs/application.js`.

Anyway, we don't need set this on build:development because assets would
be delivered from same origin in development (and w/o dev-server).