iio: imu: st_lsm6dsx: remove set but not used variable ''

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_shub.c: In function 'st_lsm6dsx_shub_read_reg':
drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_shub.c:108:41: warning:
 variable 'hub_settings' set but not used [-Wunused-but-set-variable]

It never used since introduction in commit c91c1c84 ("iio: imu: st_lsm6dsx:
add i2c embedded controller support")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>