Bug 30719: Remove unused Koha::Illbatch->requests_count method

Bug 30719: Remove unused Koha::Illbatch->requests_count method

This patch removes this stray method from the original implementation.

To test:
1. Apply this patch
2. Run:
   $ git grep requests_count
=> SUCCESS: Notice the only ILL-related mantions are API related, which
is returned by using the 'requests+count' embed, which is unrelated to
this particular method
3. Run:
   $ prove t/db_dependent/api/v1/ill_batches.t \
     t/db_dependent/Illbatches.t
=> SUCCESS: Tests pass!
4. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>