perf[Navbar]: refine style

src/views/layout/components/Navbar.vue

 <template>
-  <el-menu class="navbar" mode="horizontal">
+  <div class="navbar">
     <hamburger :toggle-click="toggleSideBar" :is-active="sidebar.opened" class="hamburger-container"/>
     <breadcrumb />
     <el-dropdown class="avatar-container" trigger="click">
@@ -18,7 +18,7 @@
         </el-dropdown-item>
       </el-dropdown-menu>
     </el-dropdown>
-  </el-menu>
+  </div>
 </template>
 
 <script>
@@ -54,7 +54,7 @@ export default {
 .navbar {
   height: 50px;
   line-height: 50px;
-  border-radius: 0px !important;
+  box-shadow: 0 1px 3px 0 rgba(0,0,0,.12), 0 0 3px 0 rgba(0,0,0,.04);
   .hamburger-container {
     line-height: 58px;
     height: 50px;