Eliminate stdout message when launching a posix thread.

This seems out of place as it's the only time RocksDB prints to stdout in the
normal course of operations. Thread IDs can still be retrieved from the LOG
file: cut -d ' ' -f2 LOG | sort | uniq | egrep -x '[0-9a-f]+'

util/env_posix.cc

@@ -1393,9 +1393,6 @@ class PosixEnv : public Env {
                          nullptr,
                          &ThreadPool::BGThreadWrapper,
                          this));
-        fprintf(stdout,
-                "Created bg thread 0x%lx\n",
-                (unsigned long)t);
 
         // Set the thread name to aid debugging
 #if defined(_GNU_SOURCE) && defined(__GLIBC_PREREQ)