Remove PyPI downloads

According to official site,
https://packaging.python.org/guides/analyzing-pypi-package-downloads/
PyPI package download statistics is no longer maintained and thus
should be removed.

Change-Id: I70c9814f8ce67a46628d2101dc638783715160db

README.rst

@@ -15,10 +15,6 @@ python-heatclient
     :target: https://pypi.python.org/pypi/python-heatclient/
     :alt: Latest Version
 
-.. image:: https://img.shields.io/pypi/dm/python-heatclient.svg
-    :target: https://pypi.python.org/pypi/python-heatclient/
-    :alt: Downloads
-
 
 OpenStack Orchestration API Client Library