Remove unnecessary unstable feature flags from examples

[wiktorpyk]

Many example files currently use unstable Rust feature flags that have been stabilized and are no longer needed:

• #![feature(type_alias_impl_trait)]
• #![feature(impl_trait_in_assoc_type)]
• #![feature(naked_functions)]

These feature flags cause compilation errors on stable Rust (1.90.0+) with:

error[E0554]: `#![feature]` may not be used on the stable release channel

Solution

Remove all three unstable feature flag declarations from the example files