IB/iser: Remove hard setting of path MTU

Remove hard setting of the IB MTU used by iSER's RC queue-pair to 1K,
as this was done due to inter-op issues with an old iser target which
is not used any more.

Signed-off-by: Or Gerlitz <ogerlitz@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>