Remove unused h-3 and w-3 classes

They were introduced in [this mr][1], with the intention of using
the. Appaarently the usage was removed during review, but the new
classes accidentally stayed around.

[1]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24724

app/assets/stylesheets/framework/common.scss

@@ -439,10 +439,6 @@ img.emoji {
 
 .min-height-0 { min-height: 0; }
 
-.w-3 { width: #{3 * $grid-size}; }
-
-.h-3 { width: #{3 * $grid-size}; }
-
 .svg-w-100 {
   svg {
     width: 100%;