gst-plugins-bad: remove libGLU dependency

libGLU is an unmaintained GL library.
glu was removed upstream in 6bd1150ca889c17342520b3bbd8e729fb843a1a5 [1],
which took effect in version 1.5.1 released June 2015 [2].

Later, in 769a21d0bb310906b880c07da0e1e2376e11c187 [3],
upstream removed the entirety of direct OpenGL integration.
This change took effect in 1.13.1, released February 2018 [4].

libGLU nix package supplies a couple shared objects,
a package config file, and a single C header.
None of these are used anymore: code search on the package
config comes back empty, and `nix why-depends` confirms
the shared objects are not being linked against.

libGLU is a dependency in many packages, but really should be phased out.

[1] https://github.com/GStreamer/gst-plugins-bad/commit/6bd1150ca889c17342520b3bbd8e729fb843a1a5
[2] https://github.com/GStreamer/gst-plugins-bad/releases/tag/1.5.1
[3] https://github.com/GStreamer/gst-plugins-bad/commit/769a21d0bb310906b880c07da0e1e2376e11c187
[4] https://github.com/GStreamer/gst-plugins-bad/releases/tag/1.13.1

pkgs/development/libraries/gstreamer/bad/default.nix

@@ -83,7 +83,6 @@
 , xvidcore
 , gnutls
 , mjpegtools
-, libGLU
 , libGL
 , addDriverRunpath
 , gtk3
@@ -250,7 +249,6 @@ stdenv.mkDerivation rec {
     sratom
 
     libGL
-    libGLU
   ] ++ lib.optionals guiSupport [
     gtk3
   ] ++ lib.optionals stdenv.hostPlatform.isDarwin [