use the base implementation hints
Summary:
with latest kirigami master and kcmutils, the default size hints
of scrollablepage are correct
also don't show empty header/footer when not needed, which do extra
useless space
Test Plan: kcmshell5 kcm_fonts starts with proper size now
Reviewers: Plasma, ngraham
Reviewed By: ngraham
Subscribers: ngraham, kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D20665