Supercat should discard existing 852 tags when asked for holdings

[Jason Boyer]

Similar to exporting holdings in marc_export, supercat should ideally not include existing 852 tags when asked for holdings data. While it is already placing the holdings data in a separate xml tag, services pulling the holdings data may potentially blend the 852 and holdings tags in an effort to avoid missing anything. (Also, inaccurate dead bits, data hygiene, mumble, mumble, etc.)

[Josh Stompro (u-launchpad-stompro-org)]

The sru_search() in SuperCat.pm does look like it tries to clear out old 852 tags before it generates new ones, so that covers sru and z39.50.

2078 $marc->delete_field($_) for ($marc->field('852')); # remove any legacy 852s

So it looks like those endpoints follow the same pattern as marc export.