WebEngine: Always disable off-the-record profiles

Previously, changing the LocalStorageEnabled setting would flip the
internal profile's off-the-record setting as well. This was most likely
a misunderstanding of what the setting is meant for (a specific HTML5
feature, not just any storage persisted on disk), and it went
unchallenged until now.

Flipping the setting can trigger odd crashes, since Chromium doesn't
really support profiles that change their off-the-recordness at
runtime. So, mark the profile as always on-the-record, and only
flip the LocalStorageEnabled setting in QQuickWebEngineProfile.

As a drive-by, remove the parent pointer inside
QWebEngineWebViewSettingsPrivate since it's no longer used and clang
complains about it.

Change-Id: Id38ef8271b685bf0ade3567f5c5dd4400815cb08
Reviewed-by: Michal Klocek <michal.klocek@qt.io>