Remove a contract

[zoedberg]

We think it would be useful to have a method to "de-register"/"forget" an asset, or in other words to remove data related to an asset from the stash. Currently AFAIK there is no way to do this. We think there could be cases where a user doesn't care anymore about an asset, he is ok with burning it, but to reduce stash data consumption (and possibly improve performance) it would be nice to have a method that allows this.
This is not urgent but a nice to have, IMO it can be done after 0.11 testing.

[dr-orlovsky]

I think we should allow removal of an asset only if the user doesn't own any state under that asset. Unlike in blockchain, undesirable airdrops are not possible (just do not accept the consignment), thus it should be enough.

[fedsten]

There may be cases where the wallet accept an incoming transfer because it is valid, but then the user wants to remove the asset because the attachment or the asset itself may be illegal or regulatory risky to own in his jurisdiction, or other cases where the user was initially happy to own the asset, but then a change in regulation made it a liability to keep owning it.

At the end there are many ways for a wallet to lose the assets of the users, so we should trust wallet developer to use this feature in a safe manner.

[zoedberg]

@dr-orlovsky can we move this to a later milestone? 0.12 for example should be ok