Turning off tree checking by default for releases

[Phil Edwards]

What's responsible for doing this?  There's no logic in configure.ac to
do it automatically, there's nothing special in gcc_release, and I don't
see any explicit reminders in releasing.html (which makes me think that
it's automatic, else we'd be reminding the RM).

-- 
"What attribute do you consider most valuable to the politician?"
"To be able to raise a cause which shall produce an effect, and
then *fight the effect*."
    - Captain T.W.S. Kidd and Senatorial candidate Abraham Lincoln