Fix api_manager failing tests about cohorts

lms/djangoapps/api_manager/users/views.py

@@ -21,6 +21,7 @@ from courseware.views import get_module_for_descriptor, save_child_position, get
 from course_groups.models import CourseUserGroup
 from course_groups.cohorts import (
     get_cohort_by_name,
+    get_cohort,
     add_cohort,
     add_user_to_cohort,
     remove_user_from_cohort
@@ -903,15 +904,9 @@ class UsersCoursesDetail(SecureAPIView):
         if not course_exists(request, user, course_id):
             return Response({}, status=status.HTTP_204_NO_CONTENT)
         course_key = get_course_key(course_id)
-        try:
-            cohort = CourseUserGroup.objects.get(
-                course_id=course_key,
-                users__id=user.id,
-                group_type=CourseUserGroup.COHORT,
-            )
+        cohort = get_cohort(user, course_key)
+        if cohort:
             remove_user_from_cohort(cohort, user.username)
-        except ObjectDoesNotExist:
-            pass
         CourseEnrollment.unenroll(user, course_key)
         return Response({}, status=status.HTTP_204_NO_CONTENT)