refactor(object-capacity): remove the decorative SymlinkTracker and its no-op depth knob (#4571)

The tracker never influenced traversal: walkdir's follow behavior is
fixed up front by follow_links(), and should_follow() only gated the
tracker's own bookkeeping. Its 'depth limit' compared tree depth (not
symlink chain depth), so RUSTFS_CAPACITY_MAX_SYMLINK_DEPTH was a
complete no-op while its telemetry claimed symlinks were skipped that
walkdir had in fact followed and counted; record_symlink was always
called with size 0, so tracked_bytes never left zero (S12).

Remove the tracker, its skipped/summary events, the symlink metric and
the depth env knob end to end (the env's 'as u8' truncation goes with
it), and document the real semantics at the walker: follow_links(true)
counts targets with walkdir's ancestor-loop detection breaking cycles,
follow_links(false) — the default — counts no symlink targets. The scan
root itself is pre-resolved since backlog#1015.

Ref: rustfs/backlog#1018 (S12 from audit rustfs/backlog#1010)

crates/io-metrics/src/lib.rs

@@ -121,7 +121,7 @@ pub use capacity_metrics::{
     record_capacity_dirty_disk_count, record_capacity_dynamic_timeout, record_capacity_refresh_inflight,
     record_capacity_refresh_joiner, record_capacity_refresh_request, record_capacity_refresh_result,
     record_capacity_refresh_scope, record_capacity_scan_disk, record_capacity_scan_mode, record_capacity_scan_sampling,
-    record_capacity_symlink, record_capacity_timeout_fallback, record_capacity_update_completed, record_capacity_update_failed,
+    record_capacity_timeout_fallback, record_capacity_update_completed, record_capacity_update_failed,
     record_capacity_write_operation, record_old_data_dir_cleanup,
 };