Remove unused args in logger module. (#517)

diff --git a/mobly/logger.py b/mobly/logger.py
index 41838e8..f82e94f 100644
--- a/mobly/logger.py
+++ b/mobly/logger.py
@@ -132,7 +132,7 @@
     return _get_timestamp('%m-%d-%Y_%H-%M-%S-%f', delta)
 
 
-def _setup_test_logger(log_path, prefix=None, filename=None):
+def _setup_test_logger(log_path, prefix=None):
     """Customizes the root logger for a test run.
 
     The logger object has a stream handler and a file handler. The stream
@@ -159,10 +159,6 @@
     ch.setLevel(logging.INFO)
     # Log everything to file
     f_formatter = logging.Formatter(log_line_format, log_line_time_format)
-    # All the logs of this test class go into one directory
-    if filename is None:
-        filename = get_log_file_timestamp()
-        utils.create_dir(log_path)
     # Write logger output to files
     fh_info = logging.FileHandler(
         os.path.join(log_path, records.OUTPUT_FILE_INFO_LOG))
@@ -210,10 +206,8 @@
         filename: Name of the files. The default is the time the objects
             are requested.
     """
-    if filename is None:
-        filename = get_log_file_timestamp()
     utils.create_dir(log_path)
-    _setup_test_logger(log_path, prefix, filename)
+    _setup_test_logger(log_path, prefix)
     logging.info('Test output folder: "%s"', log_path)
     create_latest_log_alias(log_path)
 
diff --git a/tests/mobly/output_test.py b/tests/mobly/output_test.py
index d700272..c4e9bd6 100755
--- a/tests/mobly/output_test.py
+++ b/tests/mobly/output_test.py
@@ -19,6 +19,7 @@
 import platform
 import shutil
 import tempfile
+import time
 import unittest
 import yaml
 
@@ -151,8 +152,7 @@
             info_log_path, whitelist=[info_uuid], blacklist=[debug_uuid])
 
     @mock.patch(
-        'mobly.logger.get_log_file_timestamp',
-        side_effect=['12', '12', '13', '13'])
+        'mobly.logger.get_log_file_timestamp', side_effect=str(time.time()))
     def test_run_twice_for_two_sets_of_logs(self, mock_timestamp):
         """Verifies the expected output files from a test run.
 
@@ -173,15 +173,13 @@
         self.assert_output_logs_exist(output_dir2)
 
     @mock.patch(
-        'mobly.logger.get_log_file_timestamp',
-        side_effect=['423', '423', '5435', '5435'])
+        'mobly.logger.get_log_file_timestamp', side_effect=str(time.time()))
     def test_teardown_erases_logs(self, mock_timestamp):
         """Verifies the expected output files from a test run.
 
         * Files are correctly created.
         * Basic sanity checks of each output file.
         """
-        mock_timestamp.side_effects = ['423' '5346']
         mock_test_config = self.create_mock_test_config(
             self.base_mock_test_config)
         info_uuid1 = '0c3ebb06-700d-496e-b015-62652da9e451'