Bug 17968: Remove useless variable $item_format in C4::Overdues::parse_overdues_letter

Bug 17968: Remove useless variable $item_format in C4::Overdues::parse_overdues_letter

The variable $item_format is not used and should be removed from this
subroutine.

Moreover it the letter parameter, but it is never sent to this
subroutine. letter_code is expected instead.

Test plan:
No test plan, just read the code and `git grep `

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>