hsmd: remove unused "sign_local_htlc_tx" function which onchaind used to use.

I tried removing sign_penalty_to_us, but that comment is wrong: channeld
uses that for the watchtower, so it stays (with updated comment).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

onchaind/test/run-grind_feerate-bug.c

@@ -222,12 +222,6 @@ void towire_wallet_tx_type(u8 **pptr UNNEEDED, const enum wallet_tx_type type UN
 { fprintf(stderr, "towire_wallet_tx_type called!\n"); abort(); }
 /* AUTOGENERATED MOCKS END */
 
-/* Stubs which do get called. */
-u8 *towire_hsmd_sign_local_htlc_tx(const tal_t *ctx UNNEEDED, u64 commit_num UNNEEDED, const struct bitcoin_tx *tx UNNEEDED, const u8 *wscript UNNEEDED, bool option_anchor_outputs UNNEEDED)
-{
-	return NULL;
-}
-
 u8 *wire_sync_read(const tal_t *ctx, int fd UNNEEDED)
 {
 	return towire_onchaind_spend_created(ctx, true, NULL);