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.comSigned-off-by: Eric W. Biederman <ebiederm@xmission.com>

arch/x86/kernel/doublefault_32.c

@@ -77,9 +77,6 @@ asmlinkage noinstr void __noreturn doublefault_shim(void)
 	 * some way to reconstruct CR3.  We could make a credible guess based
 	 * on cpu_tlbstate, but that would be racy and would not account for
 	 * PTI.
-	 *
-	 * Instead, don't bother.  We can return through
-	 * rewind_stack_do_exit() instead.
 	 */
 	panic("cannot return from double fault\n");
 }