Bug 35248: Refactor - Move assign_item_for_booking

Bug 35248: Refactor - Move assign_item_for_booking

This routine really belongs inside Koha::Booking rather than
Koha::Biblio. This patch moves it there, renames with _ to highlight
it's private nature.

Test plan
1) Confirm that t/db_dependant/Koha/Booking.t still passes, specifically
   the 'store' test.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit d0981505ba799b78206e9945d3bd465e0a5c5ef7)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>