style: Simplify "prohibit display: contents" adjustment.

On top of #19661.

The NAC condition is pointless because NAC don't match author rules unless they
are a pseudo-element too.

components/style/properties/properties.mako.rs

@@ -3079,11 +3079,6 @@ bitflags! {
         /// that it may affect global state, like the Device's root font-size.
         const IS_ROOT_ELEMENT = 1 << 3;
 
-        /// Whether to convert display:contents into display:inline.  This
-        /// is used by Gecko to prevent display:contents on generated
-        /// content.
-        const PROHIBIT_DISPLAY_CONTENTS = 1 << 4;
-
         /// Whether we're computing the style of a link, either visited or
         /// unvisited.
         const IS_LINK = 1 << 6;