lightningd: don't loop through all commands every time one finishes.

When we have many commands, this is where we spend all our time, and it's
just for an old assertion.

tests/test_coinmoves.py::test_generate_coinmoves (100,000, sqlite3):
	Time (from start to end of l2 node):	13 seconds **WAS 34**
	Worst latency:				4.0 seconds **WAS 24*

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

lightningd/jsonrpc.c

@@ -801,11 +801,6 @@ static struct command_result *command_exec(struct json_connection *jcon,
 	if (res == &pending)
 		assert(cmd->pending);
 
-	/* The command might outlive the connection. */
-	if (jcon)
-		list_for_each(&jcon->commands, cmd, list)
-			assert(cmd->pending);
-
 	return res;
 }