storage: remove the storage server's "remote_cancel_lease" function
We're removing this function because it is currently unused, because it is dangerous, and because the bug described in #1528 leaks the cancellation secret, which allows anyone who knows a file's storage index to abuse this function to delete shares of that file.
fixes #1528 (there are two patches that are each a sufficient fix to #1528 and this is one of them)