mb: Fully remove references to 'masters' in favor of 'builder_groups'.

All of chrome's mb_configs should have been swapped over to using
'builder_groups'.

V8's and WebRTC's copies of mb.py still have references to 'masters':
https://chromium.googlesource.com/v8/v8/+/master/tools/mb/mb.py
https://webrtc.googlesource.com/src/+/master/tools_webrtc/mb/mb.py

So updating those are left as a follow-up for the relevant project
owners.

Bug: 1117773
Change-Id: Ic58469c050cac77d834c63f4a2b6478c377d9411
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622751Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843093}

tools/mb/mb.py

@@ -121,8 +121,6 @@ class MetaBuildWrapper(object):
       group = subp.add_mutually_exclusive_group()
       group.add_argument(
           '-m',  '--builder-group',
-          # TODO(crbug.com/1117773): Remove the 'master' args.
-          '--master',
           help='builder group name to look up config from')
       subp.add_argument('-b', '--builder',
                         help='builder name to look up config from')
@@ -950,9 +948,7 @@ class MetaBuildWrapper(object):
 
     self.configs = contents['configs']
     self.mixins = contents['mixins']
-    # TODO(crbug.com/1117773): Remove 'masters' below.
-    self.builder_groups = (
-        contents.get('builder_groups') or contents.get('masters'))
+    self.builder_groups = contents.get('builder_groups')
     self.public_artifact_builders = contents.get('public_artifact_builders')
 
   def ReadIsolateMap(self):