util: Add StripRedundantLastElementsOfPath function

Summary:
This fixes a bug causing listwalletdir to strip lead char of wallet name is datadir has a trailing '/'

Co-authored-by: saibato <saibato.naga@pm.me>
Co-authored-by: MarcoFalke <falke.marco@gmail.com>

This is a backport of core#20080

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10693