Consider removing `DeleteDataSet` session key permission

[wjmelements]

The verify signature contract bytecode does not contain the DefaultFwssPermissions item DeleteDataSet (0xb5d6b3fc97881f05e96958136ac09d7e0bc7cbf17ea92fce7c431d88132d2b58). The other permissions are present. I suspect that the hash is calculated incorrectly.

[wjmelements]

After further investigation, it appears this feature has been removed. I will try to find when and why.

[wjmelements]

Ok it was removed in this PR FilOzone/filecoin-services#255

So we can remove this permission.

[rvagg]

Came up here: #618 (comment), see also filecoin-project/curio#1028 which is why we haven't just removed it yet, it may still be useful. It just shouldn't be required for anything.

[hugomrdias]

@rvagg do we want to keep this issue and this PR #694 open ?

[rvagg]

This issue and filecoin-project/curio#1028 can serve as a reminder that this has to be dealt with pre-GA in some form. For now we're not taking action but we may implement 1028, or may bail on this entirely and therefore cleanup is appropriate. Right now it's not hurting us, it's just a bit messy.