media: dvb-frontend/mxl5xx: Remove repeated verbose license text

remove it because SPDX-License-Identifier is already used

Signed-off-by: Cai Huoqing <caihuoqing@baidu.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>