No more tree closing for (most) release tagging

[Ben Hearsum]

Hi Everyone,

In the lead up to the most recent set of releases we landed an important
change to the release automation. Part of that change was enabling it to
deal with push races that may happen as part of release tagging. This
means that we no longer need to close the tree when we start a release.[1]

- Ben

[1] (Except for mobile releases, which aren't using the new code until
https://bugzilla.mozilla.org/show_bug.cgi?id=574764 is resolved.)

[crowder]

Yeah, this is outstandingly cool.