style: Don't specify font-family

Now that the default font follows the interface setting, stop
overriding it in the CSS.

https://bugzilla.gnome.org/show_bug.cgi?id=688288
6 jobs for !486 with follow-font-settings in 7 minutes and 2 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Review
passed #372969
check_commit_log

00:00:24

passed #372971
eslint

00:00:39

passed #372970
js_check

00:00:45

 
  Build
passed #372972
build

00:05:04

 
  Test
passed #372973
test

00:01:12

passed #372974
test-pot

00:01:09