trivial: debian: Don't use --parents when rmdir'ing /var/*/fwupdate

Calling 'rmdir --parents /var/cache/fwupdate' will cause it to attempt
to rmdir /var/cache and /var. Those directories are very unlikely to be
empty, so it should always quietly fail. However, there's not benefit
in attempting those removals, so let's quit doing it.

contrib/debian/fwupd.postinst

@@ -29,6 +29,6 @@ rm -f /var/lib/fwupdate/done
 rm -f /var/cache/fwupdate/done
 for dir in /var/cache/fwupdate /var/lib/fwupdate; do
 	if [ -d $dir ]; then
-	        rmdir --parents --ignore-fail-on-non-empty $dir || true
+	        rmdir --ignore-fail-on-non-empty $dir || true
 	fi
 done