fix nav-dropdown styles on mobile (vuejs#366)

* Formatted the new styl code. (vuejs#1151)

* fix nav-dropdown styles on mobile

Author: vuejs-jp-bot

themes/vue/source/css/_header.styl

@@ -55,23 +55,6 @@ body.docs
   &.team
     margin-left: 10px
 
-.nav-dropdown
-  .nav-link
-    &:hover, &.current
-      border-bottom: none
-    &.current
-      &::after
-        content: ''
-        width: 0
-        height: 0
-        border-left: 5px solid $green
-        border-top: 3px solid transparent
-        border-bottom: 3px solid transparent
-        position: absolute;
-        top: 50%;
-        margin-top: -4px;
-        left: 8px;
-
 .new-label
   position: absolute
   top: 3px