Bug 7597 - Part 2 - move sub writeoff to C4::Accounts

Bug 7597 - Part 2 - move sub writeoff to C4::Accounts

Currently, the subroutine writeoff lives in pay.pl, which
is a violation of the Koha coding guidelines, as it writes
to the database. This commit restructures and moves writeoff
to C4::Accounts::WriteOff(), and modifies pay.pl to use it.

Signed-off-by: Nicole C. Engard <nengard@bywatersolutions.com>
Tested all three patches together and was able to write off single
and multiple fines and the data was correct in the stats table.