bt: Remove unused code

This module was written for the Mastic VDAF. Mastic uses a binary tree
in two ways: (1) path building from candidate prefixes; and (2) a
breadth-first traversal for computing the evaluation proof. None of the
functionalities implemented in the `bt` module support these operations
efficiently. Instead, Mastic implements these operations directly.

Remove the unused code from the `bt` module, leaving only the data types
used by Mastic/VIDPF. It may be useful to keep these in their own
module, as the data types may eventually be useful for Poplar1/IDPF.