Remove redundant payment preimag hashing in HTLC claim pipeline

Remove redundant payment preimag hashing in HTLC claim pipeline

Currently, when we receive an HTLC claim from a peer, we first hash
the preimage they gave us before removing the HTLC, then
immediately pass the preimage to the inbound channel and hash the
preimage again before removing the HTLC and sending our peer an
`update_fulfill_htlc`. This second hash is actually only asserted
on, never used in any meaningful way as we have the htlc data
present in the same code.

Here we simply drop this second hash and move it into a
`debug_assert`.