objc_class_prefix seems useless for java projects (#304)

objc_class_prefix seems useless for java projects

Author: tsonglew

grpc-spring-boot-starter-demo/src/main/proto/route_guide.proto

@@ -16,7 +16,6 @@ syntax = "proto3";
 option java_multiple_files = true;
 option java_package = "io.grpc.examples.routeguide";
 option java_outer_classname = "RouteGuideProto";
-option objc_class_prefix = "RTG";
 
 package routeguide;