Is there a way to remove a grade such that an assignment can be graded again?
For all students currently enrolled in the course, yes.
From Course Admin>Reset
Select the Reset
Navigate to the Quizzes list, select the desired options.
and/or reset the Gradebook, select the desired options.
From Course Admin>Reset
Select the Reset
Navigate to the Quizzes list, select the desired options.
and/or reset the Gradebook, select the desired options.