style: Remove PseudoElement::inherits_all.

I plan to change servo to use all: inherit on its UA sheet.

I hope the patch below should make it good enough performance-wise. And also,
it's probably broken so I don't think it's worth supporting it specially.

Differential Revision: https://phabricator.services.mozilla.com/D8686

components/style/gecko/pseudo_element.rs

@@ -57,15 +57,6 @@ impl PseudoElement {
         PseudoElementCascadeType::Lazy
     }
 
-    /// Whether cascading this pseudo-element makes it inherit all properties,
-    /// even reset ones.
-    ///
-    /// This is used in Servo for anonymous boxes, though it's likely broken.
-    #[inline]
-    pub fn inherits_all(&self) -> bool {
-        false
-    }
-
     /// Whether the pseudo-element should inherit from the default computed
     /// values instead of from the parent element.
     ///