Remove stale doctest import

Author: jurvis

lightning-invoice/src/payment.rs

@@ -35,8 +35,6 @@
 //! #
 //! # #[cfg(feature = "no-std")]
 //! # extern crate core2;
-//! # #[cfg(feature = "no-std")]
-//! # extern crate hashbrown;
 //! #
 //! # use lightning::ln::{PaymentHash, PaymentPreimage, PaymentSecret};
 //! # use lightning::ln::channelmanager::{ChannelDetails, PaymentId, PaymentSendFailure};