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
4 jobs for !486 with follow-font-settings in 1 minute and 45 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Review
passed #336400
check_commit_log

00:00:34

 
  Build
failed #336401
build

00:01:10

 
  Test
skipped #336402
test
skipped #336403
test-pot
 
Name Stage Failure
failed
build Build
Dependency gsettings-desktop-schemas found: NO found '3.32.0' but need: '>= 3.33.0'
Did not find CMake 'cmake'
Found CMake: NO
Dependency gsettings-desktop-schemas found: NO

meson.build:91:0: ERROR: Invalid version of dependency, need 'gsettings-desktop-schemas' ['>= 3.33.0'] found '3.32.0'.

A full log can be found at /builds/fmuellner/gnome-shell/mutter/build/meson-logs/meson-log.txt
ERROR: Job failed: exit code 1