remove reference to the nightly feature from crate docs

Author: KodrAus

src/lib.rs

@@ -92,8 +92,7 @@ The `Deref` implementation uses a hidden static variable that is guarded by an a
 
 This crate provides two cargo features:
 
-- `nightly`: This uses unstable language features only available on the nightly release channel for a more optimal implementation. In practice this currently means avoiding a heap allocation per static. This feature might get deprecated at a later point once all relevant optimizations are usable from stable.
-- `spin_no_std` (implies `nightly`): This allows using this crate in a no-std environment, by depending on the standalone `spin` crate.
+- `spin_no_std`: This allows using this crate in a no-std environment, by depending on the standalone `spin` crate.
 
 Both features depend on unstable language features, which means
 no guarantees can be made about them in regard to SemVer stability.