xserver: replace XKB_BINDIR with compile time option

---

Using the configure option relieves us of the patch and passing the path
via the env var in many places. Also the env var may not be inherited
when components like gdm spawn new sessions.

pkgs/tools/misc/xvfb-run/default.nix

@@ -16,7 +16,6 @@ stdenv.mkDerivation {
 
     chmod a+x $out/bin/xvfb-run
     wrapProgram $out/bin/xvfb-run \
-      --set XKB_BINDIR "${xkbcomp}/bin" \
       --set FONTCONFIG_FILE "${fontsConf}" \
       --prefix PATH : ${stdenv.lib.makeBinPath [ getopt xorgserver xauth which utillinux gawk coreutils ]}
   '';