As discussed with Julien Lepiller on IRC, updating the PO files is not useful.
This change removes the target and associated rules that allowed updating
them. This has the benefit of not having the PO files updated automatically
when generating the .pot files, which was causing spurious changes under the
po sub-directory.
* po/doc/local.mk (make-update-po-files-rule): Remove function and its uses.
(doc-po-update): Remove target.
f13049f3a8