simplify the initialization of Portal::Grade models
simplify the initialization of Portal::Grade models

The previous implementation was erasing the existing
descriptions for Portal::Grades that were active when
the app started up.

The standard Portal::Grade models are all created by 
the rake task:

  rake app:setup:default_portal_resources
concord-consortium/rigse,concord-consortium/rigse,concord-consortium/rigse,concord-consortium/rigse,concord-consortium/rigse,concord-consortium/rigse