We're no longer providing Xenial test nodes with the switch to Nodepool
in Zuul. Remove configuration for Xenial mirroring.
I believe that we'll need to follow this up with manual cleanups to
actually remove the content with reprepro as dropping the config doesn't
lead to deleting files. This will only prevent time spent attempting to
update the files.
Change-Id: Ib07804d510d978fed3760dd97f9725bc9520e266
b78ed11b81