xap_helper: drop terms+data from `mset' command
Retrieving Xapian document terms, data (and possibly values) and
transferring to the Perl side would be an increase in complexity
and I/O both the Perl and C++ sides. It would require more I/O
in C++ and transient memory use on the Perl side where slow mset
iteration gives an opportunity to dictate memory release rate.
So lets ignore the document-related stuff here for now for
ease-of-development. We can reconsider this change if dropping
Xapian Perl bindings entirely and relying on JAOT C++ ever
becomes a possibility.