mnt: Remove dead code which might prevent from building

Clang, in particular, is not happy about dead code:

fs/namespace.c:135:37: error: unused function 'node_to_mnt_ns' [-Werror,-Wunused-function]
  135 | static inline struct mnt_namespace *node_to_mnt_ns(const struct rb_node *node)
      |                                     ^~~~~~~~~~~~~~
1 error generated.

Remove a leftover from the previous cleanup.

Fixes: 7d7d164 ("mnt: support ns lookup")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20251024132336.1666382-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: andy-shev

fs/namespace.c

@@ -132,16 +132,6 @@ EXPORT_SYMBOL_GPL(fs_kobj);
  */
 __cacheline_aligned_in_smp DEFINE_SEQLOCK(mount_lock);
 
-static inline struct mnt_namespace *node_to_mnt_ns(const struct rb_node *node)
-{
-	struct ns_common *ns;
-
-	if (!node)
-		return NULL;
-	ns = rb_entry(node, struct ns_common, ns_tree_node);
-	return container_of(ns, struct mnt_namespace, ns);
-}
-
 static void mnt_ns_release(struct mnt_namespace *ns)
 {
 	/* keep alive for {list,stat}mount() */