Reduce Ledger public members and avoid copying during RippleState lookups

While doing autobridging work, I stumbled on this and wanted to get it into develop sooner rather than later, so here we go:

Two `Ledger::getRippleStateIndex` overloads exist: one which accepts two `RippleAddress` objects and one which accepts two `Account` (a.k.a. `base_uint<160, AccountTag>`) objects.

The `Account` version converts the `Account` objects to `RippleAddress` and calls the other version. 

Meanwhile, the `RippleAddress` version calls `getAccountID` which returns an `Account` object (with the *same* value as the `Account` object you constructed the `RippleAddress` with) and then uses it directly. 

Considering that all calls to `getRippleStateIndex` but one actually pass `Account` objects directly, this seems like overkill. So this PR removes the `RippleAccount` overload and modifies the single call that used it to use the `Account` variant.