Bug 16441: Do not use a package variable to cache C4::Letters::getletter

Bug 16441: Do not use a package variable to cache C4::Letters::getletter

C4::Letters::getletter use a package variable (%letter) to cache letter
returned by the subroutine.
I have not found any direct issues caused by that but it is safer to
remove it.
It won't be a big deal to hit the DBMS to get a valid letter when
needed.

No test plan here, just confirm that the changes make sense.

Signed-off-by: Srdjan <srdjan@catalyst.net.nz>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Confirm that performance loss is just a millisecond or so per
subsequent call of getletter.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit c4f388f64966f2202e9f3e3c16dad5cef3dbe0ac)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>