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.
kde-workspace: Use automatic version (close #4383)

See https://github.com/NixOS/nixpkgs/pull/4265#discussion_r18430068

This should work but I didn't test it :grin: