Remove outdated comment in .zuul.yaml

This removes the outdated comment in .zuul.yaml. We already have not
only master branch job but also stable/queens now. And we don't have
the negative lookup anymore.

Change-Id: I8c66502832250a00c4ec1e7c89387d67fa093e0d
diff --git a/.zuul.yaml b/.zuul.yaml
index 82fb758..e8b258d 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -30,9 +30,6 @@
     name: tempest-full
     parent: devstack-tempest
     # This currently works from stable/queens on.
-    # NOTE(andreaf) Only run on master for now.
-    # The negative lookup is redudant but it's a
-    # reminder that we don't want the job running there.
     branches:
       - master
       - stable/queens