use exclusive_session to eliminate some explicit drops.

os/src/console.rs

@@ -1,6 +1,5 @@
 use crate::drivers::chardev::{CharDevice, UART};
 use core::fmt::{self, Write};
-use crate::sbi::console_putchar;
 
 struct Stdout;
 
@@ -8,7 +7,6 @@ impl Write for Stdout {
     fn write_str(&mut self, s: &str) -> fmt::Result {
         for c in s.chars() {
             UART.write(c as u8);
-            //console_putchar(c as usize);
         }
         Ok(())
     }