Remove mplayer-gui to avoid a dependency on gtk2
Closes: https://bugs.debian.org/967643
gtk2 is on its way out of Debian. Applying this change would allow mplayer to stay in Testing without interruption as gtk2 removals happen.
I guess mpv is similar to mplayer-gui and is much more popular. There are lots of video players in Debian now. GNOME has a new player named Showtime. Celluloid is a popular gtk4 app that uses libmpv instead of gstreamer (Showtime and Totem use gstreamer).