Proto: update [delegated_balance] rpc

Context

Since the introduction of frozen bonds the [delegated_balance] rpc reports an incorrect value when a delegate has frozen bonds. This MR fixes that, by computing the delegated balance as delegated balance = staking balance - full balance.

Manually testing the MR

CI.
In particular : dune exec src/proto_alpha/lib_protocol/test/integration/main.exe -- test "^frozen bonds"