Remove derivation of t-address from pubkey

[pacu]

#491 (comment)

We need to think carefully about this method, and whether it should continue to exist. It's not clear here whether the pubkey is the account pubkey or the child ("diversified") pubkey. I think my recommendation is that we get rid of transparent and Sapling derivation methods entirely, and make all access to these kinds of legacy addresses route through the unified address derivation system: basically, derive a unified address, then extract the component you want.