Drop support of old Event class name

[cedric-anne]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	-

There are many pieces of GLPI code that are using something like $itemtype::getTypeName() or new $itemtype() without previously normalizing the $itemtype value using the getItemForItemtype() method. The remaining Event itemtypes values in the database, are probably unsupported since a long time ago.
Anyway, I think the best solution here is to fix values from DB in order to remove the runtime transformation from Event to Glpi\Event.