bttv: remove unused tea5757_set_freq declaration

tea5757_set_freq has been removed since
commit 1b500373 ("[media] bttv: Convert to generic
TEA575x interface").

So tea5757_set_freq is a unused declaration, remove it.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>