mavlink_shell: fix stall on CubeOrange

On CubeOrange where no console is configured by default, starting
MAVLink shell just stalls, and doesn't work.

Also, logfile download has been reported not to work, and again, seems
to work with this change.

Signed-off-by: Julian Oes <julian@oes.ch>

src/modules/mavlink/mavlink_shell.cpp

@@ -110,8 +110,6 @@ int MavlinkShell::start()
 #ifdef __PX4_NUTTX
 	sched_lock();
 #endif /* __PX4_NUTTX */
-	fflush(stdout);
-	fflush(stderr);
 
 	int fd_backups[2]; //we don't touch stderr, we will redirect it to stdout in the startup of the shell task