get rid of manual window flags setting on controller

Summary:
with Qt 5.9, the panel controller can't get focus, so
it can't be dismissed by clicking anywhere, (couldn't
figure out what was the commit that caused the regression)
removing the manual setting of flags seems to fix it,
while the behavior seems unaltered with Qt 5.7 and 5.8

Test Plan: tested on machines with Qt 5.7, 5.8 and 5.9 installed

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: mvourlakos, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6403