README: remove deprecated google analytics beacon (#4722)

Google Universal Analytics (UA) is no longer in service.

I don't know how this was used originally.
So, I remove this for now.

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>
Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

Author: daipom

README.md

@@ -71,5 +71,3 @@ See [SECURITY](SECURITY.md) to contact us about vulnerability.
 ## Contributors:
 
 Patches contributed by [great developers](https://github.com/fluent/fluentd/contributors).
-
-[<img src="https://ga-beacon.appspot.com/UA-24890265-6/fluent/fluentd" />](https://github.com/fluent/fluentd)