fix: dropdown menu flashing #975

Author: tangjinzhou

components/dropdown/style/index.less

@@ -55,7 +55,6 @@
     border-radius: @border-radius-base;
     box-shadow: @box-shadow-base;
     background-clip: padding-box;
-    -webkit-transform: translate3d(0, 0, 0);
 
     &-item-group-title {
       color: @text-color-secondary;