[style guide] Remove section about BUG labels now that presubmit exists
[style guide] Remove section about BUG labels now that presubmit exists

The only thing better than a style guide section about something is not
even *needing* a style guide section about something :)

TBR=nduca@chromium.org

Review-Url: https://codereview.chromium.org/1825393003

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