[improvement] Picker: optimize style

packages/picker/index.less

@@ -2,7 +2,6 @@
 
 .van-picker {
   position: relative;
-  overflow: hidden;
   background-color: @picker-background-color;
   user-select: none;
   -webkit-text-size-adjust: 100%; /* avoid iOS text size adjust */
@@ -68,8 +67,8 @@
     z-index: 2;
     width: 100%;
     height: 100%;
-    background-image: linear-gradient(180deg, hsla(0, 0%, 100%, .8), hsla(0, 0%, 100%, .6)),
-      linear-gradient(0deg, hsla(0, 0%, 100%, .8), hsla(0, 0%, 100%, .6));
+    background-image: linear-gradient(180deg, hsla(0, 0%, 100%, .9), hsla(0, 0%, 100%, .4)),
+      linear-gradient(0deg, hsla(0, 0%, 100%, .9), hsla(0, 0%, 100%, .4));
     background-repeat: no-repeat;
     background-position: top, bottom;
     pointer-events: none;
@@ -89,10 +88,6 @@
       padding: 0 5px;
       color: @picker-option-text-color;
 
-      &--selected {
-        font-weight: 500;
-      }
-
       &--disabled {
         opacity: @picker-option-disabled-opacity;
       }