Don't use 'new' with jQuery.Deferred

Despite it's uppercaseness, $.Deferred is not a constructor.
It's an object factory. Adding 'new' is harmless, but confusing
and redundant. It'd be like invoking any other function that
returns an object with 'new', like `new mw.user.getRights`.

Change-Id: I9e0e02e83e699be34e92481f6117173cfe461339