QLinkedList has been moved to Qt5Compat. Remove and stop mentioning
it in docs, examples (the docs & examples for QLinkedList itself will
be moved to Qt5Compat) and remove the corresponding tests.
Also remove QT_NO_LINKED_LIST, since it's not needed anymore.
Task-number: QTBUG-81630
Task-number: QTBUG-80312
Change-Id: I4a8f1105cb60aa87e7fd67e901ec1a27c489aa31
Reviewed-by: Lars Knoll <lars.knoll@qt.io>