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

data/theme/gnome-shell-sass/_common.scss

@@ -36,10 +36,8 @@ $_hover_bg_color: lighten($bg_color,if($variant=='light', 5%, 3%));
 $_active_bg_color: if($variant == 'light', darken($bg_color, 14%), darken($bg_color, 9%));
 
 $font-size: 11;
-$font-family: Cantarell, Sans-Serif;
 
 stage {
-  font-family: $font-family;
   @include fontsize($font-size);
   color: $fg_color;
 }