Drop .center-block for .m-x-auto

- Both classes do about the same thing, but with different names
- Clarifies docs for .m-x-auto requiring a fixed width block element for it to work
- Add missing heading for clearfix section in docs (unrelated)

scss/mixins/_center-block.scss

@@ -1,7 +0,0 @@
-// Center-align a block level element
-
-@mixin center-block() {
-  display: block;
-  margin-left: auto;
-  margin-right: auto;
-}