Remove some accidental (const) markers in impls

Author: oli-obk

text/0000-const-trait-impls.md

@@ -26,7 +26,7 @@ impl const Default for () {
 struct Thing<T>(T);
 
 impl<T: (const) Default> const Default for Thing<T> {
-    (const) fn default() -> Self { Self(T::default()) }
+    const fn default() -> Self { Self(T::default()) }
 }
 
 const fn default<T: (const) Default>() -> T {
@@ -335,7 +335,7 @@ generates
 
 ```rust
 impl<T: (const) PartialEq> const PartialEq for MyStruct<T> {
-    (const) fn eq(&self, other: &Rhs) -> bool {
+    const fn eq(&self, other: &Rhs) -> bool {
         self.0 == other.0
     }
 }