iio: Remove IIO_DEV_ATTR_RESET macro

There are no users of this macro left and we have come to the conclusion that it
is not a good idea to expose the raw chip reset to userspace so the macro is
very unlikely to be used in new drivers.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>