Revert "Merge pull request #11324 from edx/ahsan/rc-add-service-user-in-cms"

This reverts commit a84b8111, reversing
changes made to 0d1765e1.

cms/envs/common.py

@@ -60,10 +60,6 @@ from lms.envs.common import (
 
     # Django REST framework configuration
     REST_FRAMEWORK,
-
-    # the following setting is referenced in a migration in openedx that gets
-    # run under both lms and cms, so it must be available in both places.
-    CREDENTIALS_SERVICE_USERNAME
 )
 from path import Path as path
 from warnings import simplefilter