staging: gdm72xx: Remove task from TODO list

Removed line related to replacement of kernel_thread with kthread,
as issue was fixed on ff5e4a1d ('Staging: gdm72xx: gdm_usb:
fix deprecated function kernel_thread')

Signed-off-by: Michalis Pappas <mpappas@fastmail.fm>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>