Refactored ccx coach tab plugin

lms/djangoapps/ccx/plugins.py

@@ -25,11 +25,12 @@ class CcxCourseTab(CourseTab):
         """
         Returns true if CCX has been enabled and the specified user is a coach
         """
-        if not user:
-            return True
         if not settings.FEATURES.get('CUSTOM_COURSES_EDX', False) or not course.enable_ccx:
+            # If ccx is not enable do not show ccx coach tab.
             return False
         if has_access(user, 'staff', course) or has_access(user, 'instructor', course):
+            # if user is staff or instructor then he can always see ccx coach tab.
             return True
+        # check if user has coach access.
         role = CourseCcxCoachRole(course.id)
         return role.has_user(user)