Remove duplicate section about mixins (#337)

source/guide.html.haml

@@ -400,38 +400,4 @@ introduction: >
     let us do something like take pixel values and convert them to percentages
     without much hassle.
 
-    ## Mixins
-
-    Some things in CSS are a bit tedious to write, especially with CSS3 and
-    the many vendor prefixes that exist. A mixin lets you make groups of CSS
-    declarations that you want to reuse throughout your site. You can even
-    pass in values to make your mixin more flexible. A good use of a mixin is
-    for vendor prefixes. Here's an example for <code>transform</code>.
-
-  - example do
-    :plain
-      @mixin transform($property) {
-        -webkit-transform: $property;
-        -ms-transform: $property;
-        transform: $property;
-      }
-
-      .box { @include transform(rotate(30deg)); }
-      ===
-      =transform($property)
-        -webkit-transform: $property
-        -ms-transform: $property
-        transform: $property
-
-      .box
-        +transform(rotate(30deg))
-
-  :markdown
-    To create a mixin you use the <code>@mixin</code> directive and give it a
-    name. We've named our mixin <code>transform</code>. We're also using
-    the variable <code>$property</code> inside the parentheses so we can pass in
-    a transform of whatever we want. After you create your mixin, you can then
-    use it as a CSS declaration starting with <code>@include</code> followed
-    by the name of the mixin.
-
     ---