Replace public operators operator==(), operator!=() of
QJsonDocument to friend methods comparesEqual().
Use QT_CORE_REMOVED_SINCE and removed_api.cpp to get rid of current
comparison methods and replace them with a friend.
Task-number: QTBUG-120300
Change-Id: I7b61765c34406b7a9fb7dd8b1fc554c87af6a3f3
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
e2bd247081