Remove warning about add_view_expr and GS import not working

Since p.dexterity version 1.0.1 it has been possible to set a custom `add_view_expr` via the fti GenericSetup xml for dexterity content types.  See https://github.com/plone/plone.dexterity/commit/d718da19526f7ef6a713995aef88ed25242a65f3