removed tutors from evaluatees in simple evaluation results

Tutors were left on the list of evaluatees for the simple
evaluation results for instructors even though they would never
be evaluated by anyone. They have now been removed.
fixed mixed evaluation results export

The export function was broken for mixed evaluation because of the
slightly different data structure that the function is given. I've
fixed it by doing slight modifications such as renaming indices
to match the ones used by simple and rubric evaluations.

I've also modified the score for evaluation_mixeval to only include
required likert questions in evaluation.php (component), which I
have already committed in my previous commit.