Remove use of once fns from std

[brson]

Once fns are experimental features that the standard library should not be using or exposing to other code: https://github.com/mozilla/rust/blob/master/src/libstd/unstable/intrinsics.rs#L334

[csainty]

This was eventually done in pull request #9863 for issue #9755 so this issue can be closed along with it.