iio: imu: st_lsm6dsx: remove st_lsm6dsx_write_with_mask() declaration

Remove st_lsm6dsx_write_with_mask() declaration since it has been removed
in commit 6674bef628e6 ("iio: imu: st_lsm6dsx: add regmap API support")

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>