[Config] Remove KDE4 config support, stop writing arbitrary config files

Summary:
The KDE4 baloofilerc was only used by dolphin in the KDE4 migration
phase and is no longer needed.

In case a baloofilerc in the legacy location does not exist, the effect
is even worse - the KConfig is instantiated with an empty path, and the
config file name is derived from the application name. This leads to
adding a "[General]" section in e.g. kcmshell5rc or systemsettingsrc.

Also see T9897.

Test Plan:
modify config using balooctl config {add,rm} or using SystemSettings
-> .config/baloorc resp. .config/systemsettingsrc is no longer written,
only .config/baloofilerc.

Reviewers: Baloo, Frameworks, poboiko, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D16392