Fix the removed counts in SIS logging

SIS logging incorrectly counted earlier removed students as removed
on every updated. Now it counts only those participants who were not
removed earlier.

course/models.py

@@ -964,11 +964,13 @@ def enroll_from_sis(self) -> Tuple[int, int]:
             # unenrolled themselves.
             students = self.all_students.filter(enrollment__from_sis=True)
             to_remove = students.exclude(student_id__in=participants)
-            qs = Enrollment.objects.filter(user_profile__in=to_remove, course_instance=self)
-            qs.update(status=Enrollment.ENROLLMENT_STATUS.REMOVED)
+            qs = (Enrollment.objects
+                  .filter(user_profile__in=to_remove, course_instance=self)
+                  .exclude(status=Enrollment.ENROLLMENT_STATUS.REMOVED)
+            )
             for e in qs:
                 invalidate_content(Enrollment, e)
-                delcount += 1
+            delcount = qs.update(status=Enrollment.ENROLLMENT_STATUS.REMOVED)
         else:
             logger.warning("%s: Received an empty participants list from SIS.", self)
             return 0, 0