fix: remove JWT_AUTH_REFRESH_COOKIE

Remove unused JWT_AUTH_REFRESH_COOKIE setting. This
setting was never actually used, so there is no timing issues.

See DEPR for details:
openedx/public-engineering#190

edx_exams/settings/base.py

@@ -238,7 +238,6 @@ def root(*path_fragments):
     'JWT_AUTH_COOKIE': 'edx-jwt-cookie',
     'JWT_AUTH_COOKIE_HEADER_PAYLOAD': 'edx-jwt-cookie-header-payload',
     'JWT_AUTH_COOKIE_SIGNATURE': 'edx-jwt-cookie-signature',
-    'JWT_AUTH_REFRESH_COOKIE': 'edx-jwt-refresh-cookie',
 }
 
 TOKEN_SIGNING = {