Remove deprecated GetLogoutNotificationContext

[josephdecock]

GetLogoutNotificationContext is marked as [Obsolete("Unused and will be removed in a future version. Use the APIs on the IUserSession directly instead.")], with a comment // TODO: remove in 7.0.

[brockallen]

On a side note, for 7.0 it might be worthwhile combing thru the codebase for other messages like this left for our future selves.

[josephdecock]

This was closed in #1457