remove unneeded rust_2018_previews

Author: steveklabnik

src/rust-2018/macros/macro-changes.md

@@ -32,7 +32,6 @@ Now, you write:
 
 ```rust,ignore
 // Rust 2018
-#![feature(rust_2018_preview)]
 
 use bar::baz;
 
@@ -68,7 +67,6 @@ Now, you write instead:
 
 ```rust,ignore
 // Rust 2018
-#![feature(rust_2018_preview)]
 use serde_derive::{Serialize, Deserialize};
 
 #[derive(Serialize, Deserialize)]

src/rust-2018/module-system/path-clarity.md

@@ -20,7 +20,7 @@ new "uniform paths" variant introduced in edition preview 2. The release of
 Rust 2018 will stabilize one of these two variants and drop the other.
 
 To test Rust 2018 with the new "uniform paths" variant, put
-`#![feature(rust_2018_preview, uniform_paths)]` at the top of your `lib.rs` or
+`#![feature(uniform_paths)]` at the top of your `lib.rs` or
 `main.rs`.
 
 Here's a brief summary: