Fix lib-button-as-link not using color argument

.lib-button-as-link(); was only using its @_link-color for disabled
variants. It was also inefficiently emitting both the default .lib-link()
hover colors, and the mixin's own provided @_link-color-hover
variant.

Let's bring both arguments into the actual .lib-link() call 
instead.

Author: Leland

lib/web/css/source/lib/_buttons.less

@@ -200,15 +200,14 @@
     .lib-css(line-height, @_line-height);
     .lib-css(margin, @_margin);
     .lib-css(padding, @_padding);
-    .lib-link();
+    .lib-link(
+        @_link-color: @_link-color,
+        @_link-color-hover: @_link-color-hover
+    );
     background: none;
     border: 0;
     display: inline;
 
-    &:hover {
-        .lib-css(color, @_link-color-hover);
-    }
-
     &:hover,
     &:active,
     &:focus {