modules/config/fonts.nix: revert "export GDFONTPATH in /etc/profile"

This reverts commit 2802538dc2e2a8923e3d3cc3e144abc4e1561760. The idea
was good, but unfortunately $GDFONTPATH has a different format, i.e.
programs that use this variable don't descend into the directories
listed here like fontconfig does, so we cannot use the same values for
both settings.