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.scss

@@ -1,5 +1,4 @@
 @import "utilities/background";
-@import "utilities/center-block";
 @import "utilities/clearfix";
 @import "utilities/pulls";
 @import "utilities/screenreaders";