firefox: remove GStreamer

Apparently, it has been dropped:

https://bugzilla.mozilla.org/show_bug.cgi?id=1234092

pkgs/applications/networking/browsers/firefox/wrapper.nix

@@ -2,7 +2,7 @@
 
 ## various stuff that can be plugged in
 , flashplayer, hal-flash
-, MPlayerPlugin, ffmpeg, gst_all, xorg, libpulseaudio, libcanberra-gtk2
+, MPlayerPlugin, ffmpeg, xorg, libpulseaudio, libcanberra-gtk2
 , jrePlugin, icedtea_web
 , trezor-bridge, bluejeans, djview4, adobe-reader
 , google_talk_plugin, fribid, gnome3/*.gnome-shell*/
@@ -67,13 +67,12 @@ let
           ++ lib.optional (cfg.enableUgetIntegrator or false) uget-integrator
           ++ extraNativeMessagingHosts
         );
-      libs = (if ffmpegSupport then [ ffmpeg ] else with gst_all; [ gstreamer gst-plugins-base ])
+      libs = lib.optional ffmpegSupport ffmpeg
             ++ lib.optional gssSupport kerberos
             ++ lib.optionals (cfg.enableQuakeLive or false)
             (with xorg; [ stdenv.cc libX11 libXxf86dga libXxf86vm libXext libXt alsaLib zlib libudev ])
             ++ lib.optional (enableAdobeFlash && (cfg.enableAdobeFlashDRM or false)) hal-flash
             ++ lib.optional (config.pulseaudio or true) libpulseaudio;
-      gst-plugins = with gst_all; [ gst-plugins-base gst-plugins-good gst-plugins-bad gst-plugins-ugly gst-ffmpeg ];
       gtk_modules = [ libcanberra-gtk2 ];
 
     in stdenv.mkDerivation {
@@ -99,7 +98,6 @@ let
       };
 
       buildInputs = [makeWrapper]
-        ++ lib.optional (!ffmpegSupport) gst-plugins
         ++ lib.optional (browser ? gtk3) browser.gtk3;
 
       buildCommand = ''
@@ -119,9 +117,7 @@ let
             --suffix PATH ':' "$out/bin" \
             --set MOZ_APP_LAUNCHER "${browserName}${nameSuffix}" \
             --set MOZ_SYSTEM_DIR "$out/lib/mozilla" \
-            ${lib.optionalString (!ffmpegSupport)
-                ''--prefix GST_PLUGIN_SYSTEM_PATH : "$GST_PLUGIN_SYSTEM_PATH"''
-            + lib.optionalString (browser ? gtk3)
+            ${lib.optionalString (browser ? gtk3)
                 ''--prefix XDG_DATA_DIRS : "$GSETTINGS_SCHEMAS_PATH" \
                   --suffix XDG_DATA_DIRS : '${gnome3.defaultIconTheme}/share'
                 ''