docs: drop Fedora info, mark Ubuntu one as Debian / Ubuntu

Newton release was long time ago. Time to retire info that we had Fedora
support.

Change-Id: I44fcb6e11b729369b5e64bdeb64e9ca7c0142979
(cherry picked from commit c186122)

Author: hrw

doc/source/admin/image-building.rst

@@ -78,11 +78,6 @@ See the :ref:`support matrix <support-matrix-base-images>` for information on
 supported base image distribution versions and supported images on each
 distribution.
 
-.. warning::
-
-   Fedora images are deprecated since Newton and will be removed
-   in the future.
-
 It is possible to build only a subset of images by specifying them on the
 command line:
 
@@ -503,8 +498,8 @@ same directory as the base Dockerfile (``kolla/docker/base``):
 
    rpm_setup_config = epel.repo,delorean.repo,delorean-deps.repo
 
-Ubuntu
-------
+Debian / Ubuntu
+---------------
 For Debian based images, additional apt sources may be added to the build as
 follows: