RDMA/rxe: Remove the unused variable obj

The member variable obj in struct rxe_task is not needed.
So remove it to save memory.

Link: https://lore.kernel.org/r/20220822011615.805603-4-yanjun.zhu@linux.dev


Signed-off-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Reviewed-by: Li Zhijian <lizhijian@fujitsu.com>
Reviewed-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>