isci: remove the completion and event state handlers

isci: remove the completion and event state handlers

With these handlers gone the rest of the state handler infrastructure is
removed.

Added some WARN_ONCEs where previously we would cause NULL pointer
dereferences or silently run handlers from a previous state.

Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>