Remove traces of the old backend system

Nicolas Fella requested to merge work/nobackends into master

KConfig used to support multiple backends, but hasn't for the entire KF5 lifetime.

Since I don't see multiple backends coming back any time soon we might as well simplify the internals.

This removes the superclass from KConfigIniBackend, renames it to KConfigIni and makes KConfig use that directly.

It also cleanes up some now unneeded stuff and deprecates the ctor that takes a backend.

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