kcms/nightcolor: Simplify pin position for manual location
We can relay on the property bindings to make the pin position always reflect the selected lat/lon in the settings.
Also make the parameter of the signal handler explicit (fixes qml warning)
This is merely a small refactor I came across when trying to reproduce and fix BUG:480892. I cannot reproduce it on Qt6.6, but sending this because, well, the job is done and maybe there's a smallish chance that reducing the number of concurrent places where we set the pin position does something.