This reverts commit c19695ab95.
Just because QSet has limited API doesn't mean we can't provide this
in an efficient way for std::unordered_set :P
Added tests.
Pick-to: 6.2
Change-Id: I4f8f0e60c810acdc666cf34f929845227ed87f3b
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>