mnt: Remove dead code which might prevent from building

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: 7d7d16498958 ("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>