Fix std::cout data race

Summary: std::cout is not thread safe. tsan complains. Eliminate it.

Test Plan: env_test with TSAN

Reviewers: yhchiang, rven, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D33087

util/env_test.cc

@@ -141,8 +141,6 @@ TEST(EnvPosixTest, TwoPools) {
       {
         MutexLock l(&mu_);
         num_running_++;
-        std::cout << "Pool " << pool_name_ << ": "
-                  << num_running_ << " running threads.\n";
         // make sure we don't have more than pool_size_ jobs running.
         ASSERT_LE(num_running_, pool_size_.load());
       }