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.

#### Potential issues
- I couldn't test with **lightdm**, as on my testing machine it fails both before and after the commit, probably due to #2065. @ocharles: you added it for lightdm in 2e088aa2; are you sure it was needed? Can someone test this with lightdm?

- I couldn't test with **bumblebee**, as my motherboard doesn't support optimus. @amiddelk: you added it in 3f1054b1e; are you sure it was needed? Can someone test this with bumblebee?