Do not delete course enrollments

when the associated CourseOverview gets recreated.

common/djangoapps/student/models.py

@@ -1162,7 +1162,7 @@ class CourseEnrollment(models.Model):
     course = models.ForeignKey(
         CourseOverview,
         db_constraint=False,
-        on_delete=models.CASCADE,
+        on_delete=models.DO_NOTHING,
     )
 
     @property