FONTCONFIG_FILE: remove setters to /etc/fonts/fonts.conf

Any reasonably new version of fontconfig does search that path by default,
and setting this globally causes problems, as 2.10 and 2.11 need
incompatible configs.

Tested: slim+xfce desktop, chrootenv-ed steam.
I have no idea why we were setting the global variable;
e.g., neither Fedora nor Ubuntu does that.

pkgs/tools/X11/bumblebee/default.nix

@@ -88,7 +88,6 @@ in stdenv.mkDerivation {
     wrapProgram "$out/sbin/bumblebeed" \
       --prefix PATH : "${commonEnv}/sbin:${commonEnv}/bin:\$PATH" \
       --prefix LD_LIBRARY_PATH : "${commonEnv}/lib:\$LD_LIBRARY_PATH" \
-      --set FONTCONFIG_FILE "/etc/fonts/fonts.conf" \
       --set XKB_BINDIR "${xorg.xkbcomp}/bin" \
       --set XKB_DIR "${xkeyboard_config}/etc/X11/xkb"