media: atomisp: Remove atomisp_gmin_find_subdev()

atomisp_gmin_find_subdev() can be used to lookup a subdev
given its i2c-adapter + i2c-client-address.

But the only caller of it reads this from the intel_v4l2_subdev_table
struct and that same struct already contains a pointer to the v4l2_subdev.

So this function is not necessary, drop it and modify its only caller
to directly take the subdev from the intel_v4l2_subdev_table struct.

Also drop struct intel_v4l2_subdev_i2c_board_info since that now no
longer is used.

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