Update file_bug after change to use only chromium.org accounts.
Update file_bug after change to use only chromium.org accounts.

This includes:
 * Adding a warning telling people to log in with their chromium.org accounts
 * Remove now-unnecessary code to guess chromium.org accounts.

BUG=catapult:#2042

Review URL: https://codereview.chromium.org/1716353003

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