combine size & square mixins into one

scss/mixins/_size.scss

@@ -1,10 +1,6 @@
 // Sizing shortcuts
 
-@mixin size($width, $height) {
+@mixin size($width, $height: $width) {
   width: $width;
   height: $height;
 }
-
-@mixin square($size) {
-  @include size($size, $size);
-}