xfce4-notifyd: use upstream-supplied configuration file (NFC)

Originally the upstream package didn't supply an xfce4-notifyd.desktop
file, so we added one locally. Now it does provide this file, so use it
instead. Committed in part to try to fix a packaging failure noted in
Joyent SunOS builds. I wasn't able to duplicate it, but this should
avoid the problem by preventing the upstream tooling from trying to
install directly to a path under PKG_SYSCONFDIR (which usually in this
example check-files.mk ignores, and we'd specified the same file name
and path to install our local version of the config file to).

(gutteridge)