Remove non-existent branches in releasenote

There is no liberty, mitaka and newton branch in kolla-ansible project.

Change-Id: I6d1ecaaea4382050bab3a8130667759bbcde3e91

releasenotes/source/index.rst

@@ -8,9 +8,6 @@ Contents
    :maxdepth: 2
 
    unreleased
-   newton
-   mitaka
-   liberty
 
 
 Indices and tables

releasenotes/source/liberty.rst

-==============================
- Liberty Series Release Notes
-==============================
-
-.. release-notes::
-   :branch: origin/stable/liberty

releasenotes/source/mitaka.rst

-===================================
- Mitaka Series Release Notes
-===================================
-
-.. release-notes::
-   :branch: origin/stable/mitaka

releasenotes/source/newton.rst

-===================================
- Newton Series Release Notes
-===================================
-
-.. release-notes::
-   :branch: origin/stable/newton