Remove releases.rst from keystone docs

The most recent update to releases.rst was made back in 2012,
but it's references in the docs page. It is providing false
(or at least, inaccurate) information. Suggest removing it
since the server side doesn't have a release related file either.

Change-Id: If2a71b55117c450afdade3cad697eba5a626263f
Merge "Remove releases.rst from keystone docs"