HPCC-8492 INDEPENDENT on dataset with child dictionary gives internal er...

...ror

An inconsistency in whether to use internal or disk serializer was causing
errors when deserializing a dataset that contained a child dictionary field.

Remove the deserializer parameter from getResultRowset so that the engine can
decide. Also cleans up the code.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>