Bug 9582: Remove unused code from members/pay.pl

Bug 9582: Remove unused code from members/pay.pl

The subroutines writeoff() and get_writeoff_sth() (and the two package
variables $writeoff_sth and $add_writeoff_sth) defined in members/pay.pl
are not used at all.

To test:
1) View members/pay.pl by looking at the record of a patron who has
   at least seven fines/fees and clicking the "Pay fines" tab after
   clicking the "Fines" tab on the left side
2) Write off a single item in the table on members/pay.pl using the
   button labelled "Write off"
3) Pay a single fine by clicking the "Pay" button
4) Pay a portion of a fine by selecting a checkbox on one of the lines
   in the table and clicking the "Pay amount" button
5) Select two lines in the table and click the "Pay selected" button
6) Write off all remaining fines

If all your actions show up in the table on the Account tab, this patch
works.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>