remove useless examples/assets/fonts/style.css (#2199)

联动 https://github.com/ElemeFE/element/issues/1703

没用到的字体，会引起编译错误，建议去除。

examples/app.vue

 <style lang="css">
   @import 'highlight.js/styles/color-brewer.css';
   @import 'assets/styles/common.css';
-  @import 'assets/styles/fonts/style.css';
 
   html, body {
     margin: 0;