iser-target: Remove interrupt coalescing

It is disabled at the moment, we will get that back
in once the target is more stable.

This reverts commit 95b60f07
"Add support for completion interrupt coalescing"

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>