ending phonon on qt4

[Harald Sitter]

Ahoi!

I am thinking of following D22164 with the complete removal of the Qt4
support in phonon. Qt4 doesn't really need extra work which is why I
opted to not remove it yet, but it does incur a heavy cost on the
readability (and by extension quality) of our cmake code.

So, using this diff as an excuse I think we could just throw Qt4 out.
Thoughts?

https://phabricator.kde.org/D22164

HS