rm responsive-invisibility mixin since it's only used once

inline the one remaining usage into .hidden-print's definition
rm now-empty scss/_utilities-responsive.scss file

[skip sauce]
[skip validator]

scss/mixins/_responsive-visibility.scss

@@ -1,7 +0,0 @@
-// Responsive utilities
-
-@mixin responsive-invisibility($parent) {
-  #{$parent} {
-    display: none !important;
-  }
-}