remove REDIS_ENABLED flag from creds

you can still use this flag locally but we have it enabled for all
environments and it doesn't need to be toggleable from credentials as it
isn't a secret value.

If we wish to turn redis off for a specific environment we can create a
PR to change the config.

pytest.ini

@@ -5,5 +5,6 @@ env =
     MMG_API_KEY=mmg-secret-key
     FIRETEXT_API_KEY=Firetext
     NOTIFICATION_QUEUE_PREFIX=testing
+    REDIS_ENABLED=0
 addopts = -p no:warnings
 xfail_strict = true