Clean up LESS files

less/lib/bootstrap/mixins/responsive-visibility.less

@@ -0,0 +1,15 @@
+// Responsive utilities
+
+//
+// More easily include all the states for responsive-utilities.less.
+.responsive-visibility() {
+  display: block !important;
+  table&  { display: table !important; }
+  tr&     { display: table-row !important; }
+  th&,
+  td&     { display: table-cell !important; }
+}
+
+.responsive-invisibility() {
+  display: none !important;
+}