Remove dependency management for Dropwizard Metrics

[wilkinsona]

I don't think we're making any direct use of our dependency management for Dropwizard Metrics. actuator-autoconfigure has an optional dependency on io.dropwizard.metrics:metrics-jmx but I don't think it's used. TBC, but it looks as if we should be able to remove our dependency management as we have no reason to constrain the version.