pc: get rid of builtin pvpanic for "-M pc-1.5"

This causes two slight backwards-incompatibilities between "-M pc-1.5"
and 1.5's "-M pc":

(1) a fw_cfg file is removed with this patch.  This is only a problem
if migration stops the virtual machine exactly during fw_cfg enumeration.

(2) after migration, a VM created without an explicit "-device pvpanic"
will stop reporting panics to management.

The first problem only occurs if migration is done at a very, very
early point (and I'm not sure it can happen in practice for reasonable-size
VMs, since it will likely take more time to send the RAM to destination,
than it will take for BIOS to scan fw_cfg).

The second problem only occurs if the guest panics _and_ has a guest
driver _and_ management knows to look at the crash event, so it is
mostly theoretical at this point in time.

Thus keep the code simple, and pretend it was never broken.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
