no-std: rm temporary extern crate std

rustls/src/lib.rs

@@ -311,7 +311,7 @@ extern crate alloc;
 // is in `std::prelude` but not in `core::prelude`. This helps maintain no-std support as even
 // developers that are not interested in, or aware of, no-std support and / or that never run
 // `cargo build --no-default-features` locally will get errors when they rely on `std::prelude` API.
-#[cfg(not(test))]
+#[cfg(all(feature = "std", not(test)))]
 extern crate std;
 
 // Import `test` sysroot crate for `Bencher` definitions.