Ghost elements are notoriously a reason of network desyncs, because they
don't get transferred over network. They are meant to be local only and
disregarded by any logic, but it is not yet the case in many places.
Upon saving, we *remove* all the ghost elements (by calling
`scenario_fix_ghosts`), based on assumption they can only be caused by
local interaction. Testing has shown there are `sv6`s in the wild that
have elements marked as ghosts and this could lead to a situation where
we strip away parts of the park. This also causes network desyncs for
reason stated above.
As we strip elements with ghost flag from saves anyway, it should be
safe to assume none of our saves introduce ghosts and this issue is
limited to hacked parks only. One example of such park is [Cocopa
Bay](http://www.nedesigns.com/park/3473/cocopa-bay/).
This change removes the flag on all map elements while importing. This
is much less invasive than removing all ghost elements on importing, as
they may contain some actual data. This ~~fixes~~ hides the desync that
could be seen in the Cocopa Bay park.
As this doesn't change any in-game logic, no network version update is
necessary.
#5094
