[libkworkspace] Simplify login1_manager_interface hack

We generate a header/cpp pair from the logind interface description

For reasons we then replace the generated cpp file with a different one, but keep the original header

Qt 6.5 changes some things in qdbusxml2cpp, which breaks this

To fix this (and simplify the whole thing) check in the generated header and remove the whole semi-autogeneration mess

The logind interface isn't likely to change in a relevant way, so it shouldn't be a problem to maintain