kernel: doc: remove outdated comment cred.c

This removes an outdated comment in prepare_kernel_cred.

There is no "cred_replace_mutex" any more, so the comment must
go away.

Signed-off-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>