git prune before checkout edx_ansible

playbooks/roles/edx_ansible/tasks/deploy.yml

 ---
+- name: set git fetch.prune to ignore deleted remote refs
+  shell: git config --global fetch.prune true
+  sudo_user: "{{ edx_ansible_user }}"
+
 - name: git checkout edx_ansible repo into edx_ansible_code_dir
   git: >
     dest={{ edx_ansible_code_dir }} repo={{ edx_ansible_source_repo }} version={{ configuration_version }}