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.scss

@@ -48,7 +48,6 @@
 
 // // Layout
 @import "mixins/clearfix";
-@import "mixins/center-block";
 // @import "mixins/navbar-align";
 @import "mixins/grid-framework";
 @import "mixins/grid";