pflash toggles mr->romd_mode. So this assert does not always hold.

1) a device was added with !mr->romd_mode, therefore effectively not
   creating a kvm slot as we want to trap every access (add = false).
2) mr->romd_mode was toggled on before remove it. There is now
   actually no slot to remove and the assert is wrong.

So let's just drop the assert.

Reported-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170920145025.19403-1-david@redhat.com>
Tested-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>