Revert font stack to be just sans-serif
We should not promote un-free fonts over their
free (as in freedom) equivalents; we used sans-serif
for many years without any visible problems, so
let's revert to it until a proper fix is found.
Partially reverts Ic5ba836364d
(cherry picked from commit 638f441adb03057a3137c10144759a48b98cad86)