fix($theme-default): IE11 doesn't support shortcut of flex

Default value of flex is `flex: 1 1 auto`, while `flex: 1` is `1 1 0`

Author: ulivz

packages/@vuepress/theme-default/components/Navbar.vue

@@ -117,8 +117,6 @@ $navbar-horizontal-padding = 1.5rem
     .search-box
       flex: 0 0 auto
       vertical-align top
-    .nav-links
-      flex 1
 
 @media (max-width: $MQMobile)
   .navbar