Deprecate and remove CreateRepositoryFromSnapshot in 17.0
The CreateRepositoryFromSnapshot RPC can be used to importa a Git repostiory from an archive. Given such an archive, Gitaly simply extracts it into place and calls it a day. This is quite scary, given that the archive can contain all kinds of misconfigurations that will ultimately cause havoc.
The last user of this RPC in Rails has been removed via Remove Bare repository import rake task (gitlab!118676 - merged). So let's deprecate and remove the RPC.