Remove register_attr feature

Signed-off-by: Yuki Okushi <jtitor@2k36.org>

compiler/rustc_feature/src/removed.rs

@@ -163,6 +163,9 @@ declare_features! (
     (removed, quad_precision_float, "1.0.0", None, None, None),
     (removed, quote, "1.33.0", Some(29601), None, None),
     (removed, reflect, "1.0.0", Some(27749), None, None),
+    /// Allows using the `#[register_attr]` attribute.
+    (removed, register_attr, "CURRENT_RUSTC_VERSION", Some(66080), None,
+     Some("removed in favor of `#![register_tool]`")),
     /// Allows using the macros:
     /// + `__diagnostic_used`
     /// + `__register_diagnostic`