IB/iser: Remove redundant is_mr indicator

It is enough to check mem_h pointer assignment, mem_h == NULL will
indicate that buffer is not registered using mr.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>