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 6 minutes and 13 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #494594
check_commit_log

00:00:29

passed #494596
eslint

00:00:58

passed #494595
js_check

00:00:29

 
  Build
passed #494597
build

00:03:00

 
  Test
passed #494598
test

00:02:13

passed #494599
test-pot

00:00:42