Kicker: Fix submenus appearing in taskmanager

Kicker Submenu still uses Dialog.

Calling staysOnTopHint explicitly is not necessary. Calling setFlags
conflicts with the KWindowSystem::setState and we end up losing the
skips taskbar.

Dialog could be changed, but it's still better to reduce code than add
workarounds on top especially with Dialog being deprecated.

BUG: 481955


(cherry picked from commit c20dcf37)