HPCC-8964 Don't generate a JFindexfromactivity keyed join for thor

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4100 from ghalliday/issue8964

HPCC-8964 Don't generate a JFindexfromactivity keyed join for thor

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>