Remove invalid `credential_callable` from authentication config

[alexmerlin]

Bug Report

config/autoload/authentication.global.php contains an array key called credential_callable which references a class method that does not exist. The key can be safely removed because the functionality it provided has been moved from src/User/src/Doctrine/UserAuthentication.php to src/Admin/src/Adapter/AuthenticationAdapter.php

Q	A
Version(s)	^4.0.0