metrics uninstalls only mostly cleanly

[broder]

metrics includes code to log when packages get installed, via /etc/apt/apt.conf.d/90debathena-metrics (or debian/90debathena-metrics in the source package), referring to a /usr/lib/debathena-metrics/install-pkg script to get run when packages are installed.

The file in /etc gets left around on uninstall (because it's a conffile), but the script in /usr gets removed. Because the apt configuration has an || : in it, this doesn't cause a failure, but it does print a message about /usr/lib/debathena-metrics/install-pkg not existing.

We could do a lightly better job in 90debathena-metrics of setting the script to something that won't print an error if the package has been uninstalled.

[broder]

screw trac formatting