[storlets] Transition wallaby, xena and yoga to EOL

Hello, As you know, stable/yoga already reached its end of maintenance phase and is now being transitioned to the unmaintained branch. Because we've not seen any attempt to backport changes to it and even older branches, I'd propose retiring yoga and older branches (wallaby and xena) instead of keeping inactive branches. Please let me know if you have any concern. -- Takashi Kajinami irc: tkajinam github: https://github.com/kajinamit launchpad: https://launchpad.net/~kajinamit