Get rid of file undeletion (aka undelete)

[crspybits]

If a user is deleted from the system, all of their files will be marked as deleted. It would seem odd to allow undeletion of those files, especially because we're requiring now that the v0 owner of a file always be the owner-- this cannot be the case for this example. A deleted user cannot be the owner of a file. It seems best to get rid of undeletion -- it was put in place only to deal with a certain kind of conflict resolution.

[crspybits]

Another case where undeletion should not be permitted: When a user removes themselves from a sharing group (or removes a sharing group). In this case too, files are marked as deleted and should not be allowed to be undeleted.