Remove label_break_value feature

[jhpratt]

This amounts to creating a loop that only runs once through.

If this feature ever stabilizes (which seems unlikely at this point), it
will be trivial to revert this to the previous state.

[jebrosen]

My async branch also removes the use of this feature, but loses the benefit of only specifying Transform::Borrowed in one place.

[jhpratt]

Yeah, I just preferred the loop to preserve Sergio's desire of only specifying it once. It's pretty trivial to inline it, though.

[jhpratt]

Closing in favor of #1060, which also removes unused feature gates.