fix: 移除重复的背景样式定义，优化样式代码

Author: IceyWu

src/uni_modules/uni-ui-plus/components/skeleton/index.scss

@@ -31,13 +31,12 @@
       width: 100%;
       height: 36rpx;
       border-radius: 3px;
+      @include background;
 
       &:first-child {
         margin-top: 0;
       }
 
-      @include background;
-
       &__list {
         width: 100%;
         display: flex;