Drop Debian PPA from openafs-client role

Now that https://bugs.debian.org/980115 has been fixed in
1.8.2-1+deb10u1 for Buster and appears in the 10.9 stable point
release (2021-03-27), we no longer need our special backport PPA of
the patched packages and are able to safely drop it from the role.

Change-Id: Id062fef9461e8f6ac66585ccf25f85a588782177

roles/openafs-client/tasks/openafs-client/Debian.yaml

@@ -11,19 +11,6 @@
     - ansible_distribution == 'Ubuntu'
   become: yes
 
-# 2021-01-20 : for now we install the ppa on debian too in order to
-# get the 1.8.6-5 era packages that fixed a timestamp rollover issue
-# (see 1.8.7 release notes).  This can likely be removed if backports
-# make it to stable; see
-# https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=980115
-- name: Install openstackci openafs PPA on Debian
-  apt_repository:
-    repo: 'ppa:openstack-ci-core/openafs'
-    codename: bionic
-  when:
-    - ansible_distribution == 'Debian'
-  become: yes
-
 - name: Install kernel headers dependency
   package:
     name: