Revert "Add support for 'padding-box' value for 'box-sizing' property (fixes #6032)"

This reverts commit 945adabd48.

The CSS Working Group resolved to drop this value from the spec:
http://log.csswg.org/irc.w3.org/css/2015-05-20/#e555680

The group was unable to come up with even a theoretical use case.
Gecko only implemented this value for completeness.
Other browsers vendors have clearly expressed they have no interest
in implementing this.

components/layout/fragment.rs

@@ -1029,10 +1029,6 @@ impl Fragment {
         }
     }
 
-    pub fn padding_width(&self) -> LogicalMargin<Au> {
-        self.border_padding - self.border_width()
-    }
-
     /// Computes the margins in the inline direction from the containing block inline-size and the
     /// style. After this call, the inline direction of the `margin` field will be correct.
     ///