Bug 16556 - KohaToMarcMapped columns sharing same field with biblio(item)number are...

Bug 16556 - KohaToMarcMapped columns sharing same field with biblio(item)number are removed.

REPLICATE ISSUE:

1. Map biblio.frameworkcode to 999$b
2. Map biblio.biblionumber to 999$c
3. Add a record with something in 999$b
4. 999$b is removed by C4::Biblio::AddBiblio()

After this patch, the field used by biblio.biblionumber or biblioitems.biblioitemnumber
is not removed and created anew, thus dropping all existing additions.

There is no point in dropping the field in any case, since we can just replace
the existing subfields in-place with no need to recreate the whole field.

UNIT TESTS INCLUDED

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit a9eb4005811882da0eb5e20b52861a3c85556dff)