Do not use multicast RPC to handle critical game data management. Multicast is suited for visual and special effects like explosions.
Your player’s inventory should be on a replicating player state or similar actor. RemoveItemServer should loop through all connected players, grab their player state, remove item from the inventory collection (if present).
This will then automatically replicate. If you need some additional things to happen on the client, when an item is removed, then use OnRep_Notify