Bug 41145: Do not delete the existing extended_attribute values when importing patrons

Since bug 26744 was upstreamed when you use the Import patron tool or the import_patrons.pl script from the commandline and pass a preserve-extended-attributes parameter the borrowerlogs inaccurately have an entry added where the attribute 'before' value is empty.

Test plan:
1. Create a patron attribute type named 'INSTID': Koha administration > Patrons and circulation > Patron attribute types > New patron attribute type.
2. Download the patron started CSV from: Tools > Import patrons > click the 'Download a Starter CSV file with all the columns.' link
3. Save values in the: cardnumber, surname, firstname, branchcode, categorycode, auth_method, lang, and patron_attributes columns
4. Make sure your value in the patron_attributes column is INSTID:1234
5. Import this csv file:
cd misc
./import_patrons.pl --file <filename>.csv --matchpoint cardnumber --default branchcode=CPL --overwrite --preserve-extended-attributes --verbose --verbose --confirm

6. Navigate to the patron account in your browser
7. Click 'Modification log' and observe how many entries there are
8. Repeat step 5
9. Notice there is a new entry in your Modification log with an info column value of: { "attribute.INSTID" : { "after" : "1234", "before" : "" } }
10. Repeat step 5 and notice another new entry in the Modification log with an empty "before" value

11. Apply patch
12. Repeat step 5 and notice no new entry in the Modification log
13. Edit your csv and change the patron_attributes value to: INSTID:5678
14. Repeat step 5 and notice the new modification log entry with an info column value of: { "attribute.INSTID" : { "after" : "5678", "before" : "1234" } }

15. Add another new patron attribute type to your Koha named 'home'
16. edit your csv and change the patron_attributes value to: INSTID:5678,home:town
17. Repeat step 5
18. Notice a new Modification log entry with the value of: { "attribute.home" : { "after" : "town", "before" : "" } }

Sponsored-by: Auckland University of Technology
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@openfifth.co.uk>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
(cherry picked from commit 2608f48952)
Signed-off-by: Jacob O'Mara <jacob.omara@openfifth.co.uk>

Koha/Patrons/Import.pm

@@ -377,9 +377,6 @@ LINE: while ( my $borrowerline = <$handle> ) {
                                 $patron_attributes =
                                     $patron->extended_attributes->merge_and_replace_with($patron_attributes);
                             }
-
-                            # We do not want to filter by branch, maybe we should?
-                            Koha::Patrons->find($borrowernumber)->extended_attributes->delete;
                             $patron->extended_attributes($patron_attributes);
                         }
                         $overwritten++;