Don't require a macro for classic portlets.

Supports the use case where you want to render a browser view in a portlet. The
classic portlet already has the necessary machinery, you just cannot configure
it through the UI without this change. Note: undid the description text change
made in
https://github.com/plone/plone.app.portlets/commit/0300730335a99e2daf7e74de883008aaae5359f6