docs: remove not-implemented-hide for statsd sink prefix (#3358)

This seems to have been implemented in
https://github.com/envoyproxy/envoy/pull/3029.

Signed-off-by: Snow Pettersen <snowp@squareup.com>

Mirrored from https://github.com/envoyproxy/envoy @ 45d3e3f64074338fb8ac4e2e41226180b9c245b9

envoy/config/metrics/v2/stats.proto

@@ -155,7 +155,7 @@ message StatsdSink {
     // Envoy will connect to this cluster to flush statistics.
     string tcp_cluster_name = 2;
   }
-  // [#not-implemented-hide:] Optional custom prefix for StatsdSink. If
+  // Optional custom prefix for StatsdSink. If
   // specified, this will override the default prefix.
   // For example:
   //