Consolidate redundant heading style declaration

[skip sauce]
[skip validator]

scss/_type.scss

@@ -4,21 +4,13 @@
 
 h1, h2, h3, h4, h5, h6,
 .h1, .h2, .h3, .h4, .h5, .h6 {
+  margin-bottom: $headings-margin-bottom;
   font-family: $headings-font-family;
   font-weight: $headings-font-weight;
   line-height: $headings-line-height;
   color: $headings-color;
 }
 
-h1, .h1,
-h2, .h2,
-h3, .h3,
-h4, .h4,
-h5, .h5,
-h6, .h6 {
-  margin-bottom: $headings-margin-bottom;
-}
-
 h1 { font-size: $font-size-h1; }
 h2 { font-size: $font-size-h2; }
 h3 { font-size: $font-size-h3; }