serial-term, radio-send: rm unused imports

beginner/apps/src/bin/radio-send.rs

@@ -2,7 +2,6 @@
 #![no_main]
 #![no_std]
 
-use cortex_m::asm;
 use cortex_m_rt::entry;
 use dk::ieee802154::{Packet, Channel, TxPower};
 use panic_log as _; // the panicking behavior

tools/serial-term/src/main.rs

@@ -1,6 +1,5 @@
 use core::sync::atomic::{AtomicBool, Ordering};
 use std::{
-    fs::OpenOptions,
     io::{self, Read as _, Write as _},
     thread,
     time::Duration,