Provide fewer default spacers

Open to suggestions on what this default scale should be, just seems
like crazy overkill to provide so many out of the box when it's so
trivial to add extra ones if you need them.

src/utilities/spacing.less

@@ -7,15 +7,6 @@
   '4' 1rem,
   '6' 1.5rem,
   '8' 2rem,
-  '12' 3rem,
-  '16' 4rem,
-  '20' 5rem,
-  '24' 6rem,
-  '32' 8rem,
-  '40' 10rem,
-  '60' 15rem,
-  '80' 20rem,
-  '120' 30rem,
 ;
 
 @padding-scale: @spacing-scale;