should remove legacy 852s during bib+holdings export

[Galen Charlton]

When exporting bib records with the option to embed items or retrieving bibs with holdings via SRU & Z39.50, any legacy 852 fields should be removed so that the consumer of the bib records does not have to try to distinguish between legacy or randomly-copy-catalogued 852 fields and those representing current items.

Attached patch implements this. Of course, there could be workflows where the library does want to pass any legacy 852s on to the consumer, in which case we could consider options to retain the legacy 852s or move them to a different field, but that would be better handled by a more general system for specifying export profiles that allow for arbitrary MARC record transformations.

[Dan Scott (denials)]

Thanks Galen - applied to trunk. Let's give it a kick in trunk and backport it if all seems well there.