package/qemu: remove comment about sh64

sh64 has been removed since commit [1]:

[1] 4338a319

Signed-off-by: Jugurtha BELKALEM <jugurtha.belkalem@smile.fr>
Reviewed-by: Thomas Huth <huth@tuxfamily.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>