Simplify usage of navigation root

[thet]

• Don't aquire navigation root via getNavigationRootObject but use getSite, which produces same results. Also fixes a problem, where related items widget options for registry controlpanel forms tried to call getPhysicalPath on a plone.app.registry RegistryProxy.
• Remove apparently unnecessary utils.get_portal_url and replace with call to getSite().absolute_url().

/cc @jensens

[jensens]

I dont think this works.

An INavigationRoot is less than a Site. https://github.com/plone/plone.app.layout/blob/master/plone/app/layout/navigation/interfaces.py#L13

Only in lineage a Childsite is both, INavigationRoot and ISite: https://github.com/collective/collective.lineage/blob/master/src/collective/lineage/utils.py#L20

But in the other common use case of plone.app.multilingual its just a INavigationRoot: https://github.com/plone/plone.app.multilingual/blob/master/src/plone/app/multilingual/content/lrf.py#L35 https://github.com/plone/plone.app.multilingual/blob/master/src/plone/app/multilingual/interfaces.py#L21