remove DEC 21154 PCI bridge

remove DEC 21154 PCI bridge

Code has not been used practically since its inception (2004)
  f2aa58c6f4a20 UniNorth PCI bridge support
or maybe even earlier, but it was consuming contributors time
as QEMU was being rewritten.
Drop it for now. Whomever would like to actually
use the thing, can make sure it actually works/reintroduce
it back when there is a user.

PS:
I've stumbled upon this when replacing PCIDeviceClass::is_bridge
field with QOM cast to PCI_BRIDGE type. Unused DEC 21154
was the only one trying to use the field with plain PCIDevice.
It's not worth keeping the field around for the sake of the code
that was commented out 'forever'.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20221129101341.185621-2-imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>