mint-arena: Fix bot level item for obelisk being start in solid

Obelisks are solid and spawned before bot level items are added. So they always start in solid.
Unlink the obelisk game entities before adding bot level items and relink afterwards.

In vanillaQ3/ioq3 this wasn't a problem because of a trace bug.
Non-brush entities use a temp box model that was hard coded to have CONTENTS_BODY.
The trace in AAS_DropToFloor uses mask CONTENTS_SOLID, so it would hit any non-brush entities.