iio: stx104: Remove unneeded struct stx104_dev code

The stx104_dev structure was used to hold private data for use in the
stx104_remove function. Now that the stx104_remove function is gone, the
stx104_dev structure and relevant code is no longer needed. This patch
removes the unnecessary code.

Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>