Eliminate public thread-pool interface

src/cpp/server/server_builder.cc

diff --git a/src/cpp/server/server_builder.cc b/src/cpp/server/server_builder.cc
index 0b11d86..db13c84 100644
--- a/src/cpp/server/server_builder.cc
+++ b/src/cpp/server/server_builder.cc
@@ -37,8 +37,8 @@
 #include <grpc/support/log.h>
 #include <grpc++/impl/service_type.h>
 #include <grpc++/server.h>
-#include <grpc++/thread_pool_interface.h>
-#include <grpc++/fixed_size_thread_pool.h>
+#include "src/cpp/server/thread_pool_interface.h"
+#include "src/cpp/server/fixed_size_thread_pool.h"
 
 namespace grpc {
 
@@ -89,10 +89,6 @@
   ports_.push_back(port);
 }
 
-void ServerBuilder::SetThreadPool(ThreadPoolInterface* thread_pool) {
-  thread_pool_ = thread_pool;
-}
-
 std::unique_ptr<Server> ServerBuilder::BuildAndStart() {
   bool thread_pool_owned = false;
   if (!async_services_.empty() && !services_.empty()) {