Remove sponsored send_inbox_message implementation (#593)

The relay-sponsored inbox send was already gated off behind a
deprecation flag. Delete the full sponsored implementation (tx builder,
nonce manager, retry loop, relay-conflict dedup, canonical-status
polling, txid recovery) rather than leaving it as dead code behind the
flag. The path was unstable: a burst of sends with an uninitialized
local nonce could wedge the relay sponsor queue (#540/#592).

send_inbox_message is retained only as a redirect to
send_inbox_message_direct so existing callers get a useful pointer
instead of an unknown-tool error. All inbox sends go through the
non-sponsored direct path.

Author: biwasxyz