style(button): remove duplicate selectors (#10160)

Author: mutoo

src/components/button/button-theme.scss

@@ -167,14 +167,6 @@
       }
     }
 
-    &.md-accent {
-      color: '{{accent-color}}';
-
-      &:hover {
-        color: '{{accent-700}}';
-      }
-    }
-
     &.md-accent {
       color: '{{accent-color}}';