Use the Arkindex S3 client in Ponos

Another follow-up of !1902 (merged): The arkindex.ponos.aws module declares a separate S3 client which uses separate Django settings, PONOS_AWS_ACCESS_KEY, PONOS_AWS_SECRET_KEY, PONOS_AWS_ENDPOINT and PONOS_AWS_REGION. Those are just set by Arkindex to be equal to the settings of the client defined in arkindex.project.aws, so we can remove ponos.aws and only use project.aws. This should simplify the Ponos tests, since many tests have to mock the clients in two or even three different ways at once.