media: atomisp: Remove unused atomisp_get_css_pipe_id() function

Remove no longer user atomisp_get_css_pipe_id() function.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>