refactor: remove c-string interfaces for DecodeBase58{Check}

Summary:
This micro-PR gets rid of base58 function interfaces that are redundant in terms of c-string / std::string variants; the c-string interface for DecodeBase58Check is completely unused outside the base58 module, while the c-string interface for DecodeBase58 is only used in unit tests, where an implicit conversion to std::string is not problematic.

This is a backport of core#19739

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, Fabien, PiRK

Reviewed By: #bitcoin_abc, Fabien, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14404