Bug 24208: Remove change calculation for writeoffs

Bug 24208: Remove change calculation for writeoffs

Bug 11373 added change calculation to the payment page.

This doesn't work very well in the case of writeoffs:
    - The "Amount paid" and "Change to give" fields don't make much
    sense in the case of writeoffs.
    - The amount for a partial writeoff has to be entered in the
    "Amount paid" field instead of the one labeled "Writeoff amount"
    just underneath.

This patch removes these unwanted fields and use the "Writeoff amount"
label on the right input.

Test plan:
1) Choose a patron who has fees.
    You can add a manual invoice if necessary.
2) Go to Accounting > Make a payment.
3) Select some fines and click "Write off selected".
4) Note the form has a lot of inputs, not all are necessary for a
writeoff.
5) Make a partial writeoff by entering a different amount in the
"Writeoff amount" field.
    => The amount written off is the amount entered in the "Amount
    paid" field.
6) Apply patch.
7) Repeat steps 1-3.
8) Notice the form only has "Amount outstanding", "Writeoff amount"
and "Note" fields, which are enough for a writeoff.
9) Entering any amount lower than or equal to the amount outstanding
in the "Writeoff amount" field should create a writeoff of the selected
amount.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>