Bug 10020: Remove DB table and fields related to ethnicity

Bug 10020: Remove DB table and fields related to ethnicity

This patch remove the ethnicity table and the ethnicity and ethnotes
fields from the 3 tables: borrowers, deletedborrowers and
borrower_modifications IF no data exist.

Test plan:
0/ Apply this patch and update the Koha::Schema
(misc/devel/update_dbix_class_files.pl).
1/ Make sure the ethnicity has at least 1 entry
2/ Launch the updatedb entry and confirm you get a warning message and
the DB structure is not updated.
3/ truncate the ethnicity table and fill ethnicity (or ethnotes) for at least 1
row of the borrowers table.
4/ Launch the updatedb entry and confirm you get a warning message and
the DB structure is not updated.
5/ Make sure the ethnicity table is empty as well as the ethniciy and
the ethnotes fields of borrowers, deletedborrowers and
borrower_modifications
6/ Launch the updatedb entry and confirm you don't get the warning
message and the DB structure is updated (ie. table and fields dropped).

Signed-off-by: Nick Clemens <nick@quecheelibrary.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>