Add mb.py to pylint check of the root PRESUBMIT.py.

This now possible because of https://webrtc-review.googlesource.com/c/src/+/256972

Bug: webrtc:13605
Change-Id: Ie5c058c5ff459dde05c90f757a1df8bd53f35ccc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/257281
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Christoffer Jansson <jansson@google.com>
Commit-Queue: Jeremy Leconte <jleconte@google.com>
Cr-Commit-Position: refs/heads/main@{#36390}

PRESUBMIT.py

@@ -937,8 +937,6 @@ def CommonChecks(input_api, output_api):
               r'^testing[\\\/].*\.py$',
               r'^third_party[\\\/].*\.py$',
               r'^tools[\\\/].*\.py$',
-              # TODO(bugs.webrtc.org/13605): should arguably be checked.
-              r'^tools_webrtc[\\\/]mb[\\\/].*\.py$',
               r'^xcodebuild.*[\\\/].*\.py$',
           ),
           pylintrc='pylintrc',