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/utilities/_center-block.scss

@@ -1,3 +0,0 @@
-.center-block {
-  @include center-block();
-}