x86/debug: Remove the historical junk

Remove the historical junk and replace it with a WARN and a comment.

The problem is that even though the kernel only uses TF single-step in
kprobes and KGDB, both of which consume the event before this, QEMU/KVM has
bugs in this area that can trigger this state so it has to be dealt with.

Suggested-by: Brian Gerst <brgerst@gmail.com>
Suggested-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20200902133201.170216274@infradead.org