Drop explicit `bitcoin_hashes` dependency in `lightning-invoice`

Drop explicit `bitcoin_hashes` dependency in `lightning-invoice`

Since `lightning-invoice` now depends on the `bitcoin` crate
directly, also depending on the `bitcoin_hashes` crate is redundant
and just means we confuse users by setting the `std` flag only on
`bitcoin`. Thus, we drop the explicit dependency here and replace
it with `bitcoin::hashes`.