xorg-server variants: eradicate symlink to /var/tmp

- Fixes #19673; it caused problems in combination with buildEnv.
- As noted, X falls back to /tmp:
  https://github.com/NixOS/nixpkgs/issues/19673#issuecomment-258871876
- Removing the directory is still required, as X would attempt to write
  into it if allowed - and probably succeed in case the user set
  nix.readOnlyStore = false; (X runs as root).
- Archeology link: 9d1569316.

pkgs/servers/x11/xorg/overrides.nix

@@ -459,8 +459,7 @@ in
           "--enable-glamor"
         ];
         postInstall = ''
-          rm -fr $out/share/X11/xkb/compiled
-          ln -s /var/tmp $out/share/X11/xkb/compiled
+          rm -fr $out/share/X11/xkb/compiled # otherwise X will try to write in it
           wrapProgram $out/bin/Xephyr \
             --add-flags "-xkbdir ${xorg.xkeyboardconfig}/share/X11/xkb"
           wrapProgram $out/bin/Xvfb \
@@ -510,7 +509,6 @@ in
         '';
         postInstall = ''
           rm -fr $out/share/X11/xkb/compiled
-          ln -s /var/tmp $out/share/X11/xkb/compiled
 
           cp -rT ${darwinOtherX}/bin $out/bin
           rm -f $out/bin/X

pkgs/servers/x11/xorg/xwayland.nix

@@ -25,7 +25,6 @@ overrideDerivation xorgserver (oldAttrs: {
 
   postInstall = ''
     rm -fr $out/share/X11/xkb/compiled
-    ln -s /var/tmp $out/share/X11/xkb/compiled
   '';
 
 }) // {