Remove GUI settings migration from Bitcoin ABC
Now that Bitcoin ABC is on a separate blockchain, it no longer makes
sense to automatically import GUI settings from Bitcoin ABC. This
simplifies the bitcoin-qt
startup process.
Background: this code was originally written by @cculianu for Bitcoin ABC in 2017 to import settings from Bitcoin Core, then forgotten, and then repurposed for Bitcoin Cash Node in 2020 to import settings from Bitcoin ABC.
Test plan: ninja check bitcoin-qt;src/qt/bitcoin-qt