Don't blank .static in ve.dm.Document

inheritClass already sets it, and blanking it is harmful if the
parent class puts anything in .static. This used to not be an
issue as ve.dm.Document's parent previously didn't use .static,
but OO.EventEmitter recently added a static method.

Change-Id: I3991a72b6b6e9968166ac7ba4aef9cee794a608a