fix(run-task): stop cleaning up fetches dir

This cleanup is problematic because it can fail if there are resources
under the fetches dir that are still being used by subprocesses spawned
by the task.

The cleanup was added ~7 years ago to workaround fetches getting re-used
on a generic-worker simple engine task. These days, everything should be
using the multiuser engine which means tasks get entirely separate
users and there's no possibility of accidentally re-using fetches
between tasks.

Also the Gecko `run-task` dropped this cleanup ~5 years ago.

Author: ahal

src/taskgraph/run-task/run-task

@@ -1340,21 +1340,14 @@ def main(args):
                 b"%s is %s\n" % (k.encode("utf-8"), os.environ[k].encode("utf-8")),
             )
 
-    try:
-        if "MOZ_FETCHES" in os.environ:
-            fetch_artifacts()
+    if "MOZ_FETCHES" in os.environ:
+        fetch_artifacts()
 
-        # Install Python requirements after fetches in case tasks want to use
-        # fetches to grab dependencies.
-        install_pip_requirements(repositories)
+    # Install Python requirements after fetches in case tasks want to use
+    # fetches to grab dependencies.
+    install_pip_requirements(repositories)
 
-        return run_command(b"task", task_args, cwd=args.task_cwd)
-    finally:
-        fetches_dir = os.environ.get("MOZ_FETCHES_DIR")
-        if fetches_dir and os.path.isdir(fetches_dir):
-            print_line(b"fetches", b"removing %s\n" % fetches_dir.encode("utf-8"))
-            remove(fetches_dir)
-            print_line(b"fetches", b"finished\n")
+    return run_command(b"task", task_args, cwd=args.task_cwd)
 
 
 if __name__ == "__main__":