refactor(tvix/castore): drop is_closed() from impl DirectoryPutter

This is only used in the gRPC version (GRPCPutter), during the test
automation.

So define it as a method there, behind #[cfg(test)], and remove from
the trait.

Change-Id: Idf170884e3a10be0e96c75d946d9c431171e5e88
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10340
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/castore/src/directoryservice/mod.rs

@@ -73,8 +73,4 @@ pub trait DirectoryPutter: Send {
     /// If there's been any invalid Directory message uploaded, and error *must*
     /// be returned.
     async fn close(&mut self) -> Result<B3Digest, Error>;
-
-    /// Return whether the stream is closed or not.
-    /// Used from some [DirectoryService] implementations only.
-    fn is_closed(&self) -> bool;
 }