Implement TF2_RemoveWearable native.

As of late there's a pretty valid usecase for having to remove wearables.

![Example botkiller fob](http://media.steampowered.com/apps/440/icons/fob_e_sniperrifle_gold.b384acaa681188fd31c069ce645933914c8fee3a.png)
TF2 botkiller weapons equip skull "fobs" to weapons that can't be cleanly removed without unequipping them from the player with a call to RemoveWearable.

Everyone seems to be implementing this function call within their own plugins, so it may be a better option to consolidate this behavior within the TF2 extension.
Does removing the wearable destroy the entity? I don't know, but I feel that the function doc should clarify.