Remove WebRTC usage of //third_party/pymock

Removes usage of Chromium's //third_party/pymock in favor of the version
provided by vpython. This is so that the third_party version can
eventually be removed.

TBR=aleloi@webrtc.org

Bug: chromium:1094489
Change-Id: I68511e11ed1e517c2b6d3bb832090a3c27e480e5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/177921
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Alex Loiko <aleloi@google.com>
Cr-Commit-Position: refs/heads/master@{#31568}

modules/audio_processing/test/py_quality_assessment/apm_quality_assessment_unittest.py

@@ -9,14 +9,9 @@
 """Unit tests for the apm_quality_assessment module.
 """
 
-import os
 import sys
 import unittest
 
-SRC = os.path.abspath(os.path.join(
-    os.path.dirname((__file__)), os.pardir, os.pardir, os.pardir))
-sys.path.append(os.path.join(SRC, 'third_party', 'pymock'))
-
 import mock
 
 import apm_quality_assessment

modules/audio_processing/test/py_quality_assessment/quality_assessment/input_mixer_unittest.py

@@ -12,14 +12,9 @@
 import logging
 import os
 import shutil
-import sys
 import tempfile
 import unittest
 
-SRC = os.path.abspath(os.path.join(
-    os.path.dirname((__file__)), os.pardir, os.pardir, os.pardir, os.pardir))
-sys.path.append(os.path.join(SRC, 'third_party', 'pymock'))
-
 import mock
 
 from . import exceptions

modules/audio_processing/test/py_quality_assessment/quality_assessment/simulation_unittest.py

@@ -12,14 +12,9 @@
 import logging
 import os
 import shutil
-import sys
 import tempfile
 import unittest
 
-SRC = os.path.abspath(os.path.join(
-    os.path.dirname((__file__)), os.pardir, os.pardir, os.pardir, os.pardir))
-sys.path.append(os.path.join(SRC, 'third_party', 'pymock'))
-
 import mock
 import pydub