tor-error: Mark define_asref_dyn_std_error as non-experimental

(I see no problems with this macro.  In the worst case, we deprecate
it someday.)

crates/tor-error/src/report.rs

@@ -96,7 +96,6 @@ impl<E: StdError + Sized + 'static> ErrorReport for E {}
 /// This trivial `AsRef` impl enables use of `tor_error::Report`.
 // Rust don't do this automatically, sadly, even though
 // it's basically `impl AsRef<dyn Trait> for T where T: Trait`.
-#[cfg(feature = "experimental-api")] // TODO HS make non-experimental, or rework
 #[macro_export]
 macro_rules! define_asref_dyn_std_error { { $ty:ty } => {
 // TODO: It would nice if this could be generated more automatically;