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

@@ -114,8 +114,6 @@ int MavlinkShell::start()
 #ifdef __PX4_NUTTX
 	sched_lock();
 #endif /* __PX4_NUTTX */
-	fflush(stdout);
-	fflush(stderr);
 
 #ifdef __PX4_POSIX
 	int remote_in_fd = dup(_shell_fds[0]);	// Input file descriptor for the remote shell