Bug 10983: Remove unused private subroutines from C4::Budgets

Bug 10983: Remove unused private subroutines from C4::Budgets

The subroutine _filter_fields is not used by the module
and the sub _columns is only used by it

This patch removes the dead code.

To test:

[1] Verify that the following tests pass

    t/Budgets.t
    t/Budgets/CanUserModifyBudget.t
    t/Budgets/CanUserUseBudget.t
    t/db_dependent/Acquisition.t
    t/db_dependent/Acquisition/GetOrdersByBiblionumber.t
    t/db_dependent/Acquisition/Invoices.t
    t/db_dependent/Acquisition/OrderFromSubscription.t
    t/db_dependent/Acquisition/TransferOrder.t
    t/db_dependent/Acquisition/close_reopen_basket.t
    t/db_dependent/Bookseller.t
    t/db_dependent/Budgets.t
    t/db_dependent/Serials.t
    t/db_dependent/Serials_2.t

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Looks good to me.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit befc07c754eb4c2199c0063f15f9ffb3f3ea8cca)
Signed-off-by: Fridolin SOMERS <fridolin.somers@biblibre.com>