clean up unnecessary z-index

close #9840

close #10385

components/checkbox/style/mixin.less

@@ -72,7 +72,6 @@
     &-input {
       position: absolute;
       left: 0;
-      z-index: 1;
       cursor: pointer;
       opacity: 0;
       top: 0;

components/input/style/mixin.less

@@ -311,20 +311,19 @@
   display: inline-block;
   width: 100%;
 
-  .@{inputClass} {
-    z-index: 1;
-  }
-
   &:hover .@{inputClass}:not(.@{inputClass}-disabled) {
     .hover();
   }
 
+  .@{inputClass} {
+    position: static;
+  }
+
   .@{inputClass}-prefix,
   .@{inputClass}-suffix {
     position: absolute;
     top: 50%;
     transform: translateY(-50%);
-    z-index: 2;
     line-height: 0;
     color: @input-color;
     :not(.anticon) {