Drop branch selection on oslo-master periodic job

Periodic jobs cannot have a branch selection. So fix
the branch to always use master.

Change-Id: Ida35e8f8fe5a631e12e77936325e557acef91a35

jenkins/jobs/python-jobs.yaml

@@ -141,7 +141,8 @@
       - print-template-name:
           template-name: "{template-name}"
       - revoke-sudo
-      - gerrit-git-prep
+      - branch-git-prep:
+          branch: master
       - shell: "/usr/local/jenkins/slave_scripts/run-tox-with-oslo-master.sh {python}"
       - assert-no-extra-files
 

zuul/layout.yaml

@@ -1143,7 +1143,6 @@ jobs:
     # This job is only for testing master of the project
     # against master of all oslo projects
   - name: ^periodic-.*-with-oslo-master$
-    branch: master
     voting: false
 
   - name: ^gate-tempest-dsvm-cells$