remove box-sizing mixin

closes #3832

Author: timoschilling

app/assets/stylesheets/active_admin/classic/components/_dropdown_menu.scss

@@ -106,11 +106,11 @@
       li {
         display: block;
         border-bottom: solid 1px #ebebeb;
-        @include box-sizing(border-box);
+        box-sizing: border-box;
 
         a {
           display: block;
-          @include box-sizing(padding-box);
+          box-sizing: padding-box;
           font-size: 0.95em;
           font-weight: bold;
           padding: 7px 16px 5px;

app/assets/stylesheets/active_admin/classic/components/_popovers.scss

@@ -78,11 +78,11 @@
     li {
       display: block;
       border-bottom: solid 1px #ebebeb;
-      @include box-sizing(border-box);
+      box-sizing: border-box;
 
       a {
         display: block;
-        @include box-sizing(padding-box);
+        box-sizing: padding-box;
         font-size: 0.95em;
         font-weight: bold;
         padding: 7px 16px 5px;

app/assets/stylesheets/active_admin/classic/mixins/_utilities.scss

@@ -8,11 +8,6 @@
   }
 }
 
-@mixin box-sizing($value: border-box) {
-  box-sizing: $value;
-}
-
-
 @mixin border-colors($top, $sides, $bottom) {
   border-color: $sides;
   border-top-color: $top;

app/assets/stylesheets/active_admin/classic/structure/_title_bar.scss

@@ -1,7 +1,7 @@
 #title_bar {
   @include section-header;
   @include clearfix;
-  @include box-sizing(border-box);
+  box-sizing: border-box;
   box-shadow: 0 1px 2px rgba(0, 0, 0, 0.37);
   display: table;
   border-bottom-color: #EEE;