xrootd: Add billing entry on delete

In contrast to other doors, the xrootd door failed to generate billing
notifications on file deletion. This patch resolves this issue.

Target: trunk
Require-notes: yes
Require-book: no
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7190/