Remove duplicate error logging

[johnkelly]

I haven't been able to figure out how to shut off the default non JSON error logging, so that the error is only logged in the JSON format instead of the default + JSON formats.

[mizalewski]

Hi!
You probably don't removed plug Plug.Logger from endpoint.ex in your project (3rd step in Installation).

[danmcclain]

The "Running with Cowboy using " message comes from here: https://github.com/phoenixframework/phoenix/blob/master/lib/phoenix/endpoint/cowboy_handler.ex#L100

[gmile]

@johnkelly apart from @mizalewski's , check your Elixir Logger's log level. I had the same issue with regular logging still appearing in logs, only to realize I had :debug level logs enabled.

Make sure that:

1. default logger is set to log only :error or :debug level,
2. plug_logger_json is set to log on info level.