i2o: Remove the dangerous kobj_to_i2o_device macro

This macro worked only when applied to variables named 'kobj'.
While this could have been fixed by simply renaming the macro argument,
a more type-safe replacement by an inline function would be preferred.
However, nobody uses this macro, so it's simpler to just remove it.

Signed-off-by: Ferenc Wagner <wferi@niif.hu>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>