style:移除vue-multiselect样式（项目中并未安装vue-multiselect plugin）

ruoyi-ui/src/assets/styles/index.scss

@@ -180,12 +180,3 @@ aside {
     margin-bottom: 10px;
   }
 }
-
-//refine vue-multiselect plugin
-.multiselect {
-  line-height: 16px;
-}
-
-.multiselect--active {
-  z-index: 1000 !important;
-}