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";

scss/_utilities.scss

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

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;
-}

scss/utilities/_center-block.scss

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