[kstyle] Drop QStyleOptionProgressBarV2 in Qt 5 style plugin

Summary:
D11566 is now in KDE Frameworks 5.45 so we can drop
QStyleOptionProgressBarV2.

Reviewers: Breeze, hpereiradacosta, broulik

Reviewed By: hpereiradacosta, broulik

Subscribers: davidedmundson, broulik, ngraham, ltoscano, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D12256