Bug 18269: Move field mappings related code to Koha::FieldMapping[s]

Bug 18269: Move field mappings related code to Koha::FieldMapping[s]

The 3 subroutines GetFieldMapping, SetFieldMapping and
DeleteFieldMapping from the C4::Biblio module were only used from the
field mappings admin page.
They can easily replaced with new packages Koha::FieldMappings based on
Koha::Object[s]

Test plan:
Add and delete field mappings (admin/fieldmapping.pl, Home ›
Administration › Keyword to MARC mapping).
Add an existing mapping > Nothing should be added

Note that this page has not been rewritten and you will not get any
feedbacks, but it's not the goal of this page to improve it.

Followed test plan, works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>