refactor: remove use of boost::algorithm::replace_first

Summary:

this trivial PR substitutes the (only) use of boost::algorithm::replace_first by a direct implementation.

This is a backport of core#20067

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13241