sphinx: Promote TLV payloads to be non-experimental

This is what provides us with the ability to add custom fields in the payload
when using `createonion` so make sure we actually have access to it.

Changelog-Changed: The TLV payloads for the onion packets are no longer considered an experimental feature and generally available.
Changelog-Added: Plugins may now handle modern TLV-style payloads via the `htlc_accepted` hook

Signed-off-by: Christian Decker <@cdecker>

common/sphinx.c

@@ -542,11 +542,6 @@ static bool sphinx_write_frame(u8 *dest, const struct sphinx_hop *hop)
 	size_t padding_size;
 	int pos = 0;
 
-#if !EXPERIMENTAL_FEATURES
-	if (hop->type != SPHINX_V0_PAYLOAD)
-		return false;
-#endif
-
 	/* Backwards compatibility for the legacy hop_data format. */
 	if (hop->type == SPHINX_V0_PAYLOAD)
 		dest[pos++] = 0x00;