You're arguing with George Lucas. Stop.
It's not "Yoda's mission." It's THE mission. The fact that you're trying to make it out to be solely Yoda's mission is HILARIOUS. Yoda and Obi-Wan were OBVIOUSLY on the same page.
You think you know everything. You don't. A quick Google search got me to that quote.
No, I'm arguing with you. As for George, he's the guy that forgot Obi-Wan had to pick up Anakin's lightsaber in RotS and had Luke make out with his sister. So stop, and engage in the argument.
Second, it's Luke's mission. Yoda, dead. Obi-Wan, dead. Yoda tries to provide Luke with the tools to decide how to handle the situation, because he's the last of the Jedi and Yoda's a whole lot wiser then Obi-Wan, without the deeply personal connection to Anakin. He knows him, but it's not the same.. Obi-Wan does not. He caught feelings. He's there for the death of Anakin. If he isn't explain his dialogue in this scene:
Luke: There's still good in him.
Obi-Wan: He's more machine now the man. Twisted and evil.
Luke: I can't do it, Ben.
Obi-Wan: You cannot escape your destiny. You must face Darth Vader again.
Luke: I can't kill my own father.
Obi-Wan: Then the Emperor has already won. You were our only hope.
How is that not Obi-Wan telling Luke to kill his papa? When Luke tells him he won't do it, he tells Luke, well that's ball game. The dark side wins, let's go home.