kmsan: drop the declaration of kmsan_save_stack()

This function is not defined anywhere.

Link: https://lkml.kernel.org/r/20250507160012.3311104-3-glider@google.com


Signed-off-by: Alexander Potapenko <glider@google.com>
Acked-by: Marco Elver <elver@google.com>
Cc: Bart van Assche <bvanassche@acm.org>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Cc: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>