[wlan][sme] Replace Tokens with oneshot::Sender in client.rs

There is no good reason for this extra abstraction. Instead of a
mpsc::unbounded<> of abstract Tokens, we can simply use
oneshot::Senders.

TEST=CQ + Manual smoke test.

Change-Id: Ibec03fae3cf85d044bf975123e821863701f8600
5 files changed