Remove a not very useful focus ring on about pages (which is only sho…

[bsclifton]

…wn around the entire body of preferences, not the individual controls)

Fixes #13992

Auditors: @petemill

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed. (Ask a Brave employee to help if you cannot access this document.)

Test Plan:

Reviewer Checklist:

• Request a security/privacy review as needed if one was not already requested.

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[bsclifton]

Focus ring was unintentionally introduced with 8597e72 (cc: @jasonrsadler)

The approach this PR takes is to hide the ring (versus removing the tabindex attribute, which may conflict with the changes @jasonrsadler made)

[jasonrsadler]

👍 lgtm

[bsclifton]

master 4f84971
0.23.x 4685993
0.22.x-release3 3f3c154