Bug 17894 - Remove and replace WriteOffFee

Bug 17894 - Remove and replace WriteOffFee

WriteOffFee is the last of the "payment" subroutines that need to be
merged into Koha::Account::pay ( as a writeoff is really just type of
payment ).

Test Plan:
1) Apply this patch
2) Verify the writeoff, and writeoff all buttons still work

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>