Fix the stuck vessel on orbital spawn.

The problem was a second joint was being created between the dock and the
root part of the spawned vessel. I suspect it might be the post-spawn
co-routines creating joints between child and parent without checking for
an existing joint (the thought hit me while thinking about this message).
Assuming that's correct, the fix in this patch is correct: simply delete
the joint and let KSP recreate it.