Removing fb-ledger from ILP Kit

[dappelt]

This issue lists some the places in the code of ILP Kit that depend on fb-ledger (please add if I missed any). To deprecate fb-ledger from ILP kit, we have to decide how to refactor each of these:

• bin/ledger.js
• api/src/lib/ledger.js
• all ledger related env. vars in bin/env.js