The entrypoint in the "openedx" Docker image was used only to define the
DJANGO_SETTINGS_MODULE environment variable, based on SERVICE_VARIANT and
SETTINGS. We ditch SETTINGS in favour of defining explicitely
DJANGO_SETTINGS_MODULE.
The problem with the Docker entrypoint is that it was bypassed whenever we ran
`tutor local exec` or `tutor k8s exec`. By removing it we make it simpler for
end-users to run manage.py commands in kubernetes.
