refactor: Remove Message.set_file() / dc_msg_set_file() and related code (#6558)

Now that we are deduplicating everywhere, we can get rid of some code.

The old python bindings did not get an optional `name` parameter because
they are deprecated anyway, but it would be easy to add it.

src/param.rs

@@ -547,7 +547,7 @@ mod tests {
 
         fs::write(fname, b"boo").await.unwrap();
         let blob = p.get_blob(Param::File, &t).await.unwrap().unwrap();
-        assert!(blob.as_file_name().starts_with("foo"));
+        assert!(blob.as_name().starts_with("$BLOBDIR/foo"));
 
         // Blob in blobdir, expect blob.
         let bar_path = t.get_blobdir().join("bar");