Remove the core bluetooth code

Remove the core bluetooth code

It's been deprecated since QEMU v3.1. We've explicitly asked in the
deprecation message that people should speak up on qemu-devel in case
they are still actively using the bluetooth part of QEMU, but nobody
ever replied that they are really still using it.

I've tried it on my own to use this bluetooth subsystem for one of my
guests, but I was also not able to get it running anymore: When I was
trying to pass-through a real bluetooth device, either the guest did
not see the device at all, or the guest crashed.

Even worse for the emulated device: When running

 qemu-system-x86_64 -bt device:keyboard

QEMU crashes once you hit a key.

So it seems like the bluetooth stack is not only neglected, it is
completely bitrotten, as far as I can tell. The only attention that
this code got during the past years were some CVEs that have been
spotted there. So this code is a burden for the developers, without
any real benefit anymore. Time to remove it.

Note: hw/bt/Kconfig only gets cleared but not removed here yet.
Otherwise there is a problem with the *-softmmu/config-devices.mak.d
dependency files - they still contain a reference to this file which
gets evaluated first on some build hosts, before the file gets
properly recreated. To avoid breaking these builders, we still need
the file around for some time. It will get removed in a couple of
weeks instead.

Message-Id: <20191120091014.16883-4-thuth@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>