Use sign_holder_htlc_transaction to sign non-anchors holder HTLCs

Use sign_holder_htlc_transaction to sign non-anchors holder HTLCs

We want to ensure we use fresh random signatures to prevent certain
classes of transaction replacement attacks at the bitcoin P2P layer.
This was already covered for commitment transactions and zero fee holder
HTLC transactions, but was missing for holder HTLC transactions on
non-anchors channels.

We can easily do this by reusing the existing
`EcdsaChannelSigner::sign_holder_htlc_transaction` method and
circumventing the existing `holder_htlc_sigs/prev_holder_htlc_sigs`
caches, which will be removed in a later commit anyway.