Remove unused BIP32 functions (pybitcointools)

[theStack]

This PR is another step towards getting rid of unused functions from the pybitcointools library which was imported in the first commit. Note that the changes are deliberately kept minimal and are divided up into multiple commit in order to make review trivial; at each commit, searching the removed function in the code-base via executing git grep funcname verifies that the function is not directly called anymore (see also commit messages).