Remove public declaration of the KWin::FPx2 debug stream operator

Qt 6 insists on this being exported, which in turns fails as this is
actually not implemented. There seemingly is an implementation, but that's
in a different namespace, so this can never have been used from the
outside anyway.

src/libkwineffects/kwinanimationeffect.h

@@ -542,8 +542,6 @@ private:
 
 } // namespace
 
-QDebug operator<<(QDebug dbg, const KWin::FPx2 &fpx2);
-
 Q_DECLARE_METATYPE(KWin::FPx2)
 Q_DECLARE_OPERATORS_FOR_FLAGS(KWin::AnimationEffect::TerminationFlags)