remove unused crate imports

such as `extern crate nb` in order to compile with
`rustc 1.22.0-nightly (744dd6c1d 2017-09-02)`

examples/claim.rs

@@ -2,7 +2,6 @@
 #![no_std]
 
 extern crate blue_pill;
-extern crate cortex_m;
 extern crate cortex_m_rtfm as rtfm;
 
 use blue_pill::stm32f103xx::Interrupt;

examples/usart1-rx-dma.rs

@@ -7,7 +7,6 @@
 
 extern crate blue_pill;
 extern crate cortex_m_rtfm as rtfm;
-extern crate nb;
 
 use blue_pill::Serial;
 use blue_pill::dma::{Buffer, Dma1Channel5};

examples/usart1-tx-dma.rs

@@ -7,7 +7,6 @@
 
 extern crate blue_pill;
 extern crate cortex_m_rtfm as rtfm;
-extern crate nb;
 
 use blue_pill::Serial;
 use blue_pill::dma::{Buffer, Dma1Channel4};