iio: adc: ina2xx: remove unused debug field from chip global data

commit 1961bce7 "iio: ina2xx: Remove trace_printk debug
statements" removed the code that used the chip->prev_ns field.
This patch cleans it up further by removing the unused field
and assignments.

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Cc: Daniel Baluta <daniel.baluta@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>