Drop menu files in favor of desktop files.

The menu files are no longer required for the Debian packages since
[CTTE #741573 Debian Menu System](https://lists.debian.org/debian-devel-announce/2015/09/msg00000.html)
which advised:

>  2. In addition to those changes, the Technical Committee resolves that
>     packages providing a .desktop file shall not also provide a menu
>     file for the same application.