remove about:preferences from web_accessible_resources

[diracdeltas]

fix #4913

Auditors: @bridiver

Test Plan:

1. open about:preferences and open devtools
2. the top-level HTTP response should not show an 'Access-Control-Allow-Origin' header

• 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).

[diracdeltas]

@feross relatedly, i noticed webtorrent exposes 'webtorrent.html' in web_accessible_resources. if this is not necessary i recommend removing it.

[diracdeltas]

merging based on #4913 (comment)