[backport#18669] log: Use Join() helper when listing log categories

Summary:
faec0638872798b58b9882ee079014555bc8393e log: Use Join() helper when listing log categories (MarcoFalke)

Pull request description:

This removes the global `ListLogCategories` and replaces it with a one-line member function `LogCategoriesString`, which just calls `Join`.

Should be a straightforward refactor to get rid of a few LOC.

Backport of Core PR18669

Test Plan:

ninja all check check-functional
./src/bitcoin-cli logging
./src/bitcoin-cli help logging

check output's alright

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8993