Resolve "Make exam a many to many field on StudentInfo model"

Closes #233 (closed)

core/models/student_info.py

@@ -7,9 +7,7 @@ from typing import Dict
 import constance
 from django.contrib.auth import get_user_model
 from django.db import models
-from django.db.models import (BooleanField, Case, F,
-                              QuerySet, Sum, Value, When)
-from django.db.models.functions import Coalesce
+from django.db.models import Sum
 
 from core.models.submission_type import SubmissionType
 from core.models.exam_type import ExamType