HPCC-9202 - DFS transaction remove/add regression

Removing then adding back the same subfile would cause a
'addSubFile: File <sub> is already a subfile of <super>' error

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Minor change following review
Merge pull request #4299 from jakesmith/hpcc-9202-3.10.8

HPCC-9202 - DFS transaction remove/add regression

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>