Hi @melship,
The only way I know of to not show the content of that box in Classic Quizzes would be to disable the "Let Students See Their Quiz Responses (Incorrect Questions Will Be Marked in Student Feedback)" option completely so students wouldn't see any feedback at all. This is basically, as you have discovered, essay questions in Classic Quizzes don't really have a "correct" or "incorrect" response to them.
Now if you have New Quizzes available and use those instead, there re more feedback options for essay questions. Your school/institution may not have New Quizzes available yet or may not encourage you to use that system, so you may want to talk to your local Canvas team.
Let us know if this helps at all.
-Chris