[buildbot] Drop mock dependency

https://bugs.webkit.org/show_bug.cgi?id=294864
rdar://154129538

Reviewed by Brianna Fan.

* Tools/CISupport/ews-build/steps.py: Import mock from unittest.

Canonical link: https://commits.webkit.org/296538@main

Author: JonWBedard

Tools/CISupport/ews-build/steps.py

@@ -29,6 +29,7 @@
 from buildbot.steps.worker import CompositeStepMixin
 from datetime import date
 from shlex import quote
+from unittest import mock
 
 from twisted.internet import defer, reactor, task
 
@@ -39,7 +40,6 @@
 from .utils import load_password, get_custom_suffix
 
 import json
-import mock
 import os
 import re
 import socket