remove some overlays no longer used

overlays/ffmpeg.nix

@@ -1,43 +0,0 @@
-self: super:
-{
-  ffmpeggit = super.ffmpeg-full.overrideAttrs (old: rec {
-    version = "4.2.2-0vhi3x2irg";
-    src = super.fetchurl {
-      url = "http://www.ffmpeg.org/releases/ffmpeg-snapshot-git.tar.bz2";
-      sha256 = "14pjy4w7a9g5dn5mssd5lzfl6pz5g8wgqz9fd4cb26dbrfb97i1b";
-    };
-    configureFlags = [
-      "--enable-gpl"
-      "--enable-version3"
-      "--disable-debug"
-      "--enable-static"
-      "--disable-ffplay"
-      "--disable-indev=sndio"
-      "--disable-outdev=sndio"
-      "--cc=gcc"
-      "--enable-fontconfig"
-      "--enable-gnutls"
-      "--enable-gmp"
-      "--enable-libgme"
-      "--enable-gray"
-      "--enable-libaom"
-      "--enable-libfribidi"
-      "--enable-libass"
-      "--enable-libfreetype"
-      "--enable-libopencore-amrnb"
-      "--enable-libopencore-amrwb"
-      "--enable-libsoxr"
-      "--enable-libopus"
-      "--enable-libtheora"
-      "--enable-libvo-amrwbenc"
-      "--enable-libwebp"
-      "--enable-libx264"
-      "--enable-libdav1d"
-    ];
-    patches = [];
-  });
-  myffmpeg = self.ffmpeggit.override (OldAttr: {
-    libaom = self.mylibaom;
-  });
-
-}

overlays/tuxkartmp.nix

@@ -1,22 +0,0 @@
-self: super:
-
-{
-  superTuxKartmp = super.superTuxKart.overrideAttrs(old: rec {
-    name = "superTuxKart-multiplayer";
-    srcs = [
-      (super.fetchFromGitHub {
-        owner	= "supertuxkart";
-        repo	= "stk-code";
-        rev	= "bb31d6b2265b86f5128f940584cee5ae3f033eaf";
-        sha256	= "0dpc3jrwdc7wv12z3hs0xc5xcdmh9i36164l46fi28bhfj9234c0";
-        name	= "stk-code";
-      })
-      (super.fetchsvn {
-        url	= "https://svn.code.sf.net/p/supertuxkart/code/stk-assets";
-        rev	= "17940";
-        sha256	= "1nhzvqh7x4jdvrck8k5xm7pvc6vyyjrdkqzlr2jnpx08mgjvgl0d";
-        name	= "stk-assets";
-      })
-    ];
-  });
-}