style: Remove -servo-text-decorations-in-effect.

It is bogus, because it depends on the display property as it's cascaded, but
the display property can change afterwards, for example, if we get blockified
because we're the root element or a flex item.

Replace it with a normal field instead.

Also, it carries some weight, because it's the last property that uses this
concept of "derived" property, and "custom cascade". So we can remove some code
after this.

Compute it after the cascade process in StyleAdjuster.

components/layout/display_list/builder.rs

@@ -2172,9 +2172,8 @@ impl FragmentDisplayListBuilding for Fragment {
         }
 
         // Create display items for text decorations.
-        let text_decorations = self.style()
-            .get_inheritedtext()
-            ._servo_text_decorations_in_effect;
+        let text_decorations =
+            self.style().get_inheritedtext().text_decorations_in_effect;
 
         let stacking_relative_content_box = LogicalRect::from_physical(
             self.style.writing_mode,