sd-json: stop printing debug messages about extension fields

sd-json: stop printing debug messages about extension fields

The intent was good, but we now print two or three of those messages
for each report metrics received on the wire. If the json object is
extensible, then it's all good and we don't need to inundate the user
with this trivial information. (And the message also sounds like
something is wrong or unexpected, when it totally isn't.)

...
(string):1:73: Unrecognized object field 'object', assuming extension.
(string):1:89: Unrecognized object field 'value', assuming extension.
json-stream: Received message: {"parameters":{"name":"io.systemd.Network.CarrierState","object":"virbr0","value":"degraded-carrier"},"continues":true}
(string):1:66: Unrecognized object field 'object', assuming extension.
(string):1:83: Unrecognized object field 'value', assuming extension.
json-stream: Received message: {"parameters":{"name":"io.systemd.Network.CarrierState","object":"lo","value":"carrier"},"continues":true}
(string):1:66: Unrecognized object field 'object', assuming extension.
(string):1:79: Unrecognized object field 'value', assuming extension.
json-stream: Received message: {"parameters":{"name":"io.systemd.Network.CarrierState","object":"wlp0s20f3","value":"carrier"},"continues":true}
(string):1:66: Unrecognized object field 'object', assuming extension.
(string):1:86: Unrecognized object field 'value', assuming extension.
...