x86/crash: address Misra 2.1 rule violation

The use of unreachable(), when unreachability is visible to Eclair (and
compilers), is deemed a violation. Drop the redundant statement.

No functional change.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/crash.c

@@ -118,8 +118,6 @@ static int cf_check do_nmi_crash(
 
     for ( ; ; )
         halt();
-
-    unreachable();
 }
 
 static void nmi_shootdown_cpus(void)