cronet: Delete TODO for User-Agent on CronetEngine

gRPC doesn't create the CronetEngine, so even though streaming is
observing the CronetEngine's User-Agent, we don't have control of that.
In addition, CronetEngines are commonly shared between gRPC and normal
HTTP traffic, so we don't actually expect users to set gRPC in engine's
user agent. The existing behavior seems to be working as well as
feasible.

Fixes grpc#11582

Author: Sangamesh1997

cronet/src/main/java/io/grpc/cronet/CronetClientStream.java

@@ -362,7 +362,6 @@ private static boolean isApplicationHeader(String key) {
   private void setGrpcHeaders(BidirectionalStream.Builder builder) {
     // Psuedo-headers are set by cronet.
     // All non-pseudo headers must come after pseudo headers.
-    // TODO(ericgribkoff): remove this and set it on CronetEngine after crbug.com/588204 gets fixed.
     builder.addHeader(USER_AGENT_KEY.name(), userAgent);
     builder.addHeader(CONTENT_TYPE_KEY.name(), GrpcUtil.CONTENT_TYPE_GRPC);
     builder.addHeader("te", GrpcUtil.TE_TRAILERS);