Refactor EraseLastKElements to be a special case of EraseLastKElementsIf
Refactor EraseLastKElements to be a special case of EraseLastKElementsIf

Summary:
This combines  `EraseLastKElementsIf` (introduced in D9863) with the existing `EraseLastKElements` function by using a default `return true` predicate.

This is a backport of [[https://github.com/bitcoin/bitcoin/pull/20197 | core#20197]] [5/6]
https://github.com/bitcoin/bitcoin/pull/20197/commits/8f1a53eb027727a4c0eaac6d82f0a8279549f638
Depends on D10978

Test Plan:
`ninja all check-all`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10979

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