Replace usage of deprecated SlaveBase::config() by SlaveBase::configValue

Summary:
With D23523 and D24582, we can avoid some IO by not using SlaveBase::config()

Will wait for KF 5.64 release before landing.

Test Plan: Compiles

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: elvisangelaccio, kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D24686