ehea: kill unused ehea_rereg_mr_task

ehea_rereg_mr_task is not used.  Remove it and drop @work parameter
from ehea_rereg_mrs().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Breno Leitao <leitao@linux.vnet.ibm.com>
Cc: netdev@vger.kernel.org