MB: Remove unnecessary configs.

webrtc/build/mb_config.pyl

Index: webrtc/build/mb_config.pyl
diff --git a/webrtc/build/mb_config.pyl b/webrtc/build/mb_config.pyl
index aadc3876aa9f83d767ed764da6415780473b4acc..9fd3c764eeeaf2b8a844e1a8be5a9ef38fb2df86 100644
--- a/webrtc/build/mb_config.pyl
+++ b/webrtc/build/mb_config.pyl
@@ -128,10 +128,4 @@
       'ios64_gn_rel': 'ios_gn_release_bot_arm64_device',
     },
   },
-
-  # These lists are assumed to exist so we need them in order to be able to run
-  # the mb.py script in PRESUBMIT.py to verify our config is sane.
-  'common_dev_configs': [],
-  'private_configs': [],
-  'unsupported_configs': [],
 }