Deletion of Deprecated ObjectiveAssessment.LearningObjective reference

LearningObjective was previously deprecated in favor of a hierarchical organization of learning standards. This reference should instead use GradebookEntry.LearningStandard.

As we are making breaking changes to the assessment 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)