Remove dbus-crossroads and code using it

That library didn't do it for me.
Adding async methods was a pain in the a** with lifetime and type bounds
confusing the heck out of me.
So I tried it with `dbus::tree` (in the next commits). It still was a
bit of work to get it working, but at least it **is** working.

sfsmcd/Cargo.toml

@@ -12,6 +12,5 @@ tokio = { version = "0.2", features = ["full"] }
 async-minecraft-ping = "0.2"
 dbus = { version = "0.8", features = ["futures"] }
 dbus-tokio = "0.5"
-dbus-crossroads = "0.2"
 log = "0.4"
 env_logger = "0.7"