HPCC-9609 - Tidy up dataLinkStart calls

Unecessary calls passing in activity string/id, when CThorDataLink
already had activity with same info.

Also resolve a uninitialized warning picked up by Valgrind.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #4589 from jakesmith/hpcc-9609

HPCC-9609 - Tidy up dataLinkStart calls

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>