Remove setting of version/release from releasenotes

Releasenotes are version independent, so remove version/release display.
This also allows to build the document without nova installed.

Change-Id: Iadb57089a866ef4b334670456a4116c245dbd968

releasenotes/source/conf.py

@@ -52,11 +52,10 @@ bug_tag = ''
 # |version| and |release|, also used in various other places throughout the
 # built documents.
 #
-from nova.version import version_info as nova_version
-# The short X.Y version.
-version = nova_version.canonical_version_string()
+# Release notes are version independent, no need to set version and release.
+version = ""
 # The full version, including alpha/beta/rc tags.
-release = nova_version.version_string_with_vcs()
+release = ""
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.