Fix Python worker prometheus exporter

[dandavison]

The prometheus server is started by the core bridge nowadays. This PR gets rid of some draft code we had that was starting a server directly in Python. Without this PR there are errors in the python code and a port conflict that can be reproduced with

docker build -f dockerfiles/python.Dockerfile -t omes-python .
docker run -e RUST_BACKTRACE=full omes-python \
        --scenario workflow_with_single_noop_activity \
        --run-id local-test-run \
        --language python \
        --namespace default \
        --server-address=host.docker.internal:7233 \
        --worker-prom-listen-address=127.0.0.1:9090