iio: mlx90614: Drop unused register macros

To simplify addition of support for new parts, drop unused register macros.
No functional change.

Acked-by: Crt Mori <cmo@melexis.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20230510102251.10118-3-marex@denx.de


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>