Remove all conditional checks for Qt 4.x

[vladimiroff]

Now that #2558 has been closed there is no point of having to check whether Qt is <5.0.

[CLAassistant]

All committers have signed the CLA.

[michaelstingl]

#2558 has been replaced by #5470 (2.3.1 release)

[ogoffart]

We still have to wait a bit before we can totally stop supporting Qt 4.
Then we will have to decide what's the minimal version of Qt we support. And we can also drop all the old Qt 5.x.

[guruz]

@vladimiroff This has merge conflicts now, could you re-base?

[guruz]

thanks @vladimiroff