Docs: remove past sponsors and old blog post link

Leave this page only for sponsors.
We do have another page that explains adverstising in a better way.

Author: humitos

docs/user/sponsors.rst

@@ -15,39 +15,11 @@ Current sponsors
   and make Read the Docs a better service for scientific projects.
 * You? (Email us at [email protected] for more info)
 
-Past sponsors
--------------
-
-* `Microsoft Azure`_
-* `Python Software Foundation`_
-* `Revsys`_
-* `Mozilla Web Dev`_
-* `Django Software Foundation`_
-* Lab305
-* `Twilio`_
-* `Rackspace`_
-* `Mozilla`_
-
-.. _Revsys: https://www.revsys.com/
-.. _Python Software Foundation: https://www.python.org/psf/
-.. _Mozilla Web Dev: https://blog.mozilla.org/webdev/
-.. _Django Software Foundation: https://www.djangoproject.com/foundation/
-.. _Rackspace: https://www.rackspace.com/
-.. _Mozilla: https://www.mozilla.org/en-US/
-.. _Twilio: https://www.twilio.com/
 .. _Cloudflare: https://www.cloudflare.com/
-.. _Microsoft Azure: https://azure.microsoft.com/
 .. _AWS: https://aws.amazon.com/
 .. _Chan Zuckerberg Initiative: https://chanzuckerberg.com/
 
 
-Sponsorship information
------------------------
-
-As part of increasing sustainability,
-Read the Docs is testing out promoting sponsors on documentation pages.
-We have more information about this in our `blog post <https://blog.readthedocs.com/ads-on-read-the-docs/>`_ about this effort.
-
 Sponsor us
 ~~~~~~~~~~