media: atomisp: Remove empty atomisp_css_set_cont_prev_start_time() function

atomisp_css_set_cont_prev_start_time() is a no-op, remove it.

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