Deletion of Deprecated GradebookEntry.LearningObjective reference

LearningObjective is deprecated in favor of a hierarchical organization of learning standards. This reference should instead be GradebookEntry.LearningStandard.

As we are making breaking changes to the domain, this entity will be removed.

Note that this is an important piece of cleanup: the current model forces unnecessary complexity of providers and consumers of the data. 

AC

• Remove the element
• Check for and remove sample data (unlikely)
• Should be removed from the domain file as well