Remove unneeded imports in r2d2 doctest
//! # use diesel::r2d2::CustomizeConnection;
//! # use diesel::r2d2::Error as R2D2Error;
//! use diesel::r2d2::Pool;
-//! use diesel::r2d2::PooledConnection;
//! use diesel::result::Error;
-//! use std::env;
//! use std::thread;
//!
//! # #[derive(Copy, Clone, Debug)]
