x86/mce: Remove explicit/superfluous tracing

There's some explicit tracing left in exc_machine_check_kernel(),
remove it, as it's already implied by irqentry_nmi_enter().

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210106144017.719310466@infradead.org