Remove dockerhub link from readme file

As docker changed their open-source policy, and we need to run through a complex apply process, we decided to sunset our dockerhub repo and only provide container images via quay.io.

README.md

@@ -1,7 +1,6 @@
 # Python programmable Prometheus exporter
 
 [![Docker Repository on Quay](https://quay.io/repository/codeaffen/p3exporter/status "Docker Repository on Quay")](https://quay.io/repository/codeaffen/p3exporter)
-[![Docker Image Version](https://img.shields.io/docker/v/codeaffen/p3exporter?label=dockerhub&logo=dockerhub&sort=semver)](https://hub.docker.com/r/codeaffen/p3exporter)
 [![PyPI version](https://badge.fury.io/py/p3exporter.svg)](https://badge.fury.io/py/p3exporter)
 [![Codacy Badge](https://app.codacy.com/project/badge/Grade/0c608f1a8a18412ba2031853b8963be7)](https://www.codacy.com/gh/codeaffen/p3exporter/dashboard?utm_source=github.com&utm_medium=referral&utm_content=codeaffen/p3exporter&utm_campaign=Badge_Grade)
 [![Documentation Status](https://readthedocs.org/projects/p3exporter/badge/?version=develop)](https://p3exporter.readthedocs.io/en/latest/?badge=latest)