Drop deprecated code from Less files

less/mixins/hide-text.less

@@ -1,21 +1,8 @@
 // CSS image replacement
-//
-// Heads up! v3 launched with with only `.hide-text()`, but per our pattern for
-// mixins being reused as classes with the same name, this doesn't hold up. As
-// of v3.0.1 we have added `.text-hide()` and deprecated `.hide-text()`.
-//
-// Source: https://github.com/h5bp/html5-boilerplate/commit/aa0396eae757
-
-// Deprecated as of v3.0.1 (will be removed in v4)
-.hide-text() {
+.text-hide() {
   font: ~"0/0" a;
   color: transparent;
   text-shadow: none;
   background-color: transparent;
   border: 0;
 }
-
-// New mixin to use as of v3.0.1
-.text-hide() {
-  .hide-text();
-}