Remove Qt 4 support.

This has persisted long enough, and as far as I am aware, has no users. It is
also blocking potential build system cleanups and other good things.

Reviewed-by: Valerio Valerio <valerio.valerio@jollamobile.com>
Reviewed-by: Matthew Vogt <matthew.vogt@qinetic.com.au>
Change-Id: I5e0e0549ef6ffee31b5e60ec94a9e17f27e01436
Reviewed-by: John Brooks <john.brooks@dereferenced.net>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>