Remove no longer needed extern crate proc_macro

Author: taiki-e

futures-macro/src/lib.rs

@@ -8,11 +8,6 @@
     )
 ))]
 
-// Since https://github.com/rust-lang/cargo/pull/7700 `proc_macro` is part of the prelude for
-// proc-macro crates, but to support older compilers we still need this explicit `extern crate`.
-#[allow(unused_extern_crates)]
-extern crate proc_macro;
-
 use proc_macro::TokenStream;
 
 mod executor;