refcell_replace_swap: remove feature gate & obsolete documentation item

src/libcore/cell.rs

@@ -702,8 +702,6 @@ impl<T> RefCell<T> {
     /// Replaces the wrapped value with a new one computed from `f`, returning
     /// the old value, without deinitializing either one.
     ///
-    /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
-    ///
     /// # Panics
     ///
     /// Panics if the value is currently borrowed.

src/libsyntax/feature_gate.rs

@@ -703,8 +703,6 @@ declare_features! (
     (accepted, extern_crate_self, "1.34.0", Some(56409), None),
     // support for arbitrary delimited token streams in non-macro attributes
     (accepted, unrestricted_attribute_tokens, "1.34.0", Some(55208), None),
-    // add replace and swap functions to RefCell
-    (accepted, refcell_replace_swap, "1.35.0", Some(43570), None),
 );
 
 // If you change this, please modify `src/doc/unstable-book` as well. You must