Introduce EraseLastKElementsIf to simplify the node eviction code

Summary:
There is a redundant pattern in the SelectNodeToEvict algorithm, which can be fatorized to make the code easier to follow.

There is no change in behavior.

Ref T1634.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1634

Differential Revision: https://reviews.bitcoinabc.org/D9863