Cleanup bench/base58.cpp
Cleanup bench/base58.cpp

Summary:
Inspired by Core PR 10735.
Related to T419.

Test Plan: make check && bench_bitcoin

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1857

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