FUSE filesystems don't need an fsync handler

If a FUSE filesystem's backend makes every call durable before returning, you can drop fsync from the backend API. In libfuse, leaving .fsync unset makes the kernel get ENOSYS on its first FUSE_FSYNC; it then remembers no_fsync and reports success for all later fsync() calls. Alternatively implement .fsync as return 0; to skip that first round trip. Before removing the API call, grep benchmarks and tools that time it: dropping it can change their latency-sample counts (for example a separate fsync latency field). Update any JSON consumers that read that field.