media: cx231xx: remove a now unused var

drivers/media/usb/cx231xx/cx231xx-dvb.c: In function 'unregister_dvb':
drivers/media/usb/cx231xx/cx231xx-dvb.c:607:21: warning: unused variable 'client' [-Wunused-variable]
  struct i2c_client *client;
                     ^~~~~~

Fixes: a12481354656 ("media: cx231xx: Switch to using new dvb i2c helpers")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>