IMPALA-1240: TestCancellationFullSort hits mem limit of 300MB with tpch_parquet

The test sets a mem_limit of 300MB and does:

   delay(t)
   assert(query_state != EXCEPTION)
   cancel()

If the mem_limit is hit before the delay completes, the assert will fail.  So,
the test is racy.  It appears we are right on the 300 MB memory limit (on my
machine, the query completes with peak mem usage in the high 200 MB).

I'm not sure why the mem_limit dimension was added, but given that it makes
the test racy and doesn't seem to add value, let's remove it and re-enable the
test.

Change-Id: I63767161bec6d7d6cd3de2e4563425297ffccb0d
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4366
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins