perf(button): Remove extra CSS, now that ripple handles tap highlight color (#1520)

Author: lynnmercier

packages/mdc-button/_mixins.scss

@@ -88,7 +88,6 @@
   text-align: center;
   user-select: none;
   -webkit-appearance: none;
-  -webkit-tap-highlight-color: rgba(0, 0, 0, 0);
   overflow: hidden;
   vertical-align: middle;