It's been deprecated since QEMU v3.1.0. Time to finally remove it now.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20191205104109.18680-1-thuth@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  Reworked Thomas's deprecated.texi to the rst