[media] cx231xx: remove some unused functions

This file came originally from cx23885 driver. Some functions aren't
used. Now that they are declared as static, we have those errors:

drivers/media/video/cx231xx/cx231xx-417.c:615: warning: ‘mc417_gpio_set’ defined but not used
drivers/media/video/cx231xx/cx231xx-417.c:625: warning: ‘mc417_gpio_clear’ defined but not used
drivers/media/video/cx231xx/cx231xx-417.c:635: warning: ‘mc417_gpio_enable’ defined but not used

As they're not used, just remove them. If needed, they can be restored from
the git logs or from the cx23885 driver.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>