refactor(core): deprecate hdPath in src/travelRule.ts
refactor(core): deprecate hdPath in src/travelRule.ts

Also remove `self` rebinding.

The fact that there is a `this.bitgo.decrypt` call further down makes
me doubt this was ever called.

Issue: BG-34381

BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS