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

Bug: 63512
Change-Id: Ibcdaff0174d526c5411aa88aea8b1dbc088372ee