Remove old file when rbxm fallback triggers

rojo-test/syncback-test-snapshots/end_to_end__rojo_test__syncback_util__rbxm_fallback.snap

@@ -1,5 +1,6 @@
 ---
 source: tests/rojo_test/syncback_util.rs
+assertion_line: 45
 expression: "visualize_fs_snapshot(&fs_snapshot, &output_path)"
 ---
 added_files:
@@ -9,4 +10,5 @@ added_dirs:
   - ReplicatedStorage/ChildWithoutDuplicates
   - ReplicatedStorage/ChildWithoutDuplicates/Child
 removed_files: []
-removed_dirs: []
+removed_dirs:
+  - ReplicatedStorage/ChildWithDuplicates

src/syncback/mod.rs

@@ -199,6 +199,11 @@ pub fn syncback_loop(
         let syncback = match middleware.syncback(&snapshot) {
             Ok(syncback) => syncback,
             Err(err) if middleware == Middleware::Dir => {
+                if snapshot.old_inst().is_some() {
+                    // We need to remove the old FS representation if we're
+                    // reserializing it as an rbxm.
+                    fs_snapshot.remove_dir(&snapshot.path);
+                }
                 let new_middleware = match env::var(DEBUG_MODEL_FORMAT_VAR) {
                     Ok(value) if value == "1" => Middleware::Rbxmx,
                     Ok(value) if value == "2" => Middleware::JsonModel,