media: saa7134: remove unused declarations in saa7134.h

saa7134_mixer_fops and saa7134_dsp_fops have been removed
in media/video since commit 166fb6b4 ("V4L/DVB (6623):
remove saa7134-oss").

Then media/video has been renamed to media/pci since
commit b285192a ("[media] rename most media/video pci
drivers to media/pci").

saa7134_vbi_template has been removed since
commit a9622391 ("V4L/DVB (6792): Fix VBI support").

so saa7134_mixer_fops, saa7134_dsp_fops and saa7134_vbi_template
are unused declarations, remove them.

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