iio: st_accel_core: Remove unneeded define

Definition of ST_SENSORS_WAI_ADDRESS was introduced within a very
first commit of this driver, but it was never used.
This address is already defined as ST_SENSORS_DEFAULT_WAI_ADDRESS
in include/linux/iio/common/st_sensors.h

To avoid duplication of the same constant in two different places
called almost exactly the same, the one which was never used
should be removed.

Signed-off-by: Robert Kmiec <robert.r.kmiec@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>