*: use getPortalObject directly

Products.CMFCore's version 2.7.0 of portal_url.getPortalObject
returns the portal in an incomplete acquisition context, which
causes issues when the site is registered as ISiteRoot utility
(the typical error being AttributeError REQUEST)

ERP5Site.getPortalObject works just fine and is used in many
places, this commit replaces all occurences of
portal_url.getPortalObject by getPortalObject.