Drop Prometheus middleware from dramatiq settings

Prometheus client became optional in dramatiq v2:
https://github.com/Bogdanp/dramatiq/releases/tag/v2.0.0

I think we don't need to cover all our dependencies's subdependencies, so it is fine to drop it.

Author: amureki

tests/testapp/settings.py

@@ -132,7 +132,6 @@
 DRAMATIQ_BROKER = {
     "BROKER": os.getenv("DRAMATIQ_BROKER", "dramatiq.brokers.redis.RedisBroker"),
     "MIDDLEWARE": [
-        "dramatiq.middleware.Prometheus",
         "dramatiq.middleware.AgeLimit",
         "dramatiq.middleware.TimeLimit",
         "dramatiq.middleware.Callbacks",