pc_piix: remove pc-i440fx-1.4 up to pc-i440fx-1.7

These are the last users of the 128K SeaBIOS blob in the i440FX family.
Removing them allows us to drop PCI support from the 128K blob,
thus making it easier to update SeaBIOS to newer versions.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>