pci-hotplug-old: Has been dead for five major releases, bury

Commit 79ca616 (v1.6.0) accidentally disabled legacy x86-only HMP
commands pci_add, pci_del: it defined CONFIG_PCI_HOTPLUG only as make
variable, not as preprocessor macro, killing the code conditional on
defined(CONFIG_PCI_HOTPLUG_OLD).

In all this time, nobody reported the loss.  I only noticed it when I
tried to test some error reporting change that forced me to touch this
old crap again.

Fun: git-log hw/pci/pci-hotplug-old.c shows our faith in the backward
compatibility god has been strong enough to sacrifice at its altar
about a dozen times, but not strong enough to even once verify the
legacy feature's still there, let alone works.

Remove the commands along with the code backing them.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
