IMPALA-1010: Remove Close() of build side in blocking join node.

This optimization is generally not safe since the probe side is still streaming. The
join node could acquire all of the data from the child into its own pool but then
there's no real point in doing this (doesn't lead to lower memory footprint and just
makes the mem accounting harder to reason about).

This is exposed in busy plans.

Change-Id: I37b0f6507dc67c79e5ebe8b9242ec86f28ddad41
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2722
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins