bug 2297: improve ModBiblio() to avoid duplicate item fields

bug 2297: improve ModBiblio() to avoid duplicate item fields

Prior to this patch, ModBiblio() would append
item tags from the previous version of the bib record
to the incoming bib record before saving the results,
even if the incoming bib record already has embedded
item tags.

For example, if a bib is retrieved using GetMarcBiblio() then
saved using ModBiblio(), the caller was obliged
to delete any item tags first to avoid duplication.

ModBiblio() now deletes item tags supplied in the
incoming MARC record.  This eliminates the possibility
of duplication, and removes any implication that
ModBiblio() can or should be used to modify item
records - ModItem() should be used for that.

Signed-off-by: Joshua Ferraro <jmf@liblime.com>