Fix SUB_Remove deferred thinks

[Rainyan]

Fix #330

Previously, we were overriding SUB_Remove for NT guns as a hack to prevent them from disappearing mid-round. This fix re-enables the correct SUB_Remove behaviour, and instead handles the deferred removals at the SetPickupTouch override, based on mp_weaponstay cvar's boolean value - with the exception of the ghost never being deleted from the world, regardless of the cvar's value.

We're defaulting to the more sensible mp_weaponstay 1 behaviour to keep dropped guns in the world over parity, but the original NT behaviour can be restored by toggling this cvar to 0 instead.

[Rainyan]

Geh, should've squashed it. Oh well.