[skip ci] Revert adding horovod rank in log messages

This reverts commit 04feeae774c081b2faf1a50cc2ea75049f21f12c.

ml-agents/mlagents/logging_util.py

 import logging
-import horovod.tensorflow as hvd
-    horovod_worker = "worker-%s " % str(hvd.rank())
-    log_format = horovod_worker + "%(asctime)s %(levelname)s [%(filename)s:%(lineno)d] %(message)s"
+    log_format = "%(asctime)s %(levelname)s [%(filename)s:%(lineno)d] %(message)s"
 
     logging.basicConfig(level=log_level, format=log_format, datefmt=date_format)
     logger = logging.getLogger(name=name)