Bug 18782: Remove unused C4::Serials::getsupplierbyserialid

Bug 18782: Remove unused C4::Serials::getsupplierbyserialid

TEST PLAN
----------
git grep -i getsupplierby
-- only the code removed and the test tweaked
git bz apply 18782
sudo koha-shell -c bash kohadev
prove -v t/db_dependent/Serials.t
qa -v 2 c 1
exit
-- sign off

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
(cherry picked from commit c69e02c4419a2d4a7ca31af6441081cadd4e51b3)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>