DolphinContextMenu: Do not check if the clicked folder is in the Places

This prevents that the setup of the PlacesItemModel queries Solid for
the available devices, which can take some time.

Places can be added multiple times now using the context menu, but this
has always been possible with drag and drop anyway.

See https://forum.kde.org/viewtopic.php?f=223&t=130617 for a dicsussion
on this topic.

REVIEW: 126904