exit/doublefault: Remove apparently bogus comment about rewind_stack_do_exit

I do not see panic calling rewind_stack_do_exit anywhere, nor can I
find anywhere in the history where doublefault_shim has called
rewind_stack_do_exit.  So I don't think this comment was ever actually
correct.

Cc: Andy Lutomirski <luto@kernel.org>
Fixes: 7d8d8cfd

 ("x86/doublefault/32: Rewrite the x86_32 #DF handler and unify with 64-bit")
Link: https://lkml.kernel.org/r/20211020174406.17889-1-ebiederm@xmission.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>