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This removes every reference but the "--master/-m" cmd-line arg and the
"masters" mb_config.pyl key, which will be removed in a follow-up once
all users of mb.py (ie: recipes) have switched over.

"builder_group" is also the term we're using when replacing "master" in
recipe code: crbug.com/1109276. So we should conform on using that term
going forward.

Bug: 1117773
Change-Id: I10a3c7a4bc5fd5146bc88d670564a745cb3fce2a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2605225
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#840641}