Remove tab icons for about:newtab and about:blank

[cezaraugusto]

• 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).

After tweaking the UI for tabs with no icon, it also addressed a fix for #5431

Fix #5776
Fix #5431

Auditors: @bsclifton

/cc @BradRichter could you checkout and see if the fix included here for #5431 fits our need?

Screenshot for #5431:

Test Plan (#5776):

• Automated test should pass
• npm run test -- --grep="tabs with icons"

Manual test (Test Plan for #5431):

• Open 10 tabs with icons
• Resize width to minimum possible
• Close button should stay fit to its parent

[bsclifton]

Manually tested; looks and works great! I was a little scared of the tab fix (the disable button issue), but it worked good in every scenario that I tried 😄

[bsclifton]

Stay tuned; this PR may be causing an issue... Will be looking into it, trying to confirm. The issue can be seen here:

I suspected it may be this because of the change w/ the close button

[luixxiul]

@cezaraugusto would you please update the manual test plan for #5431 explained in the first post, if it is no longer valid?

[bbondy]

about:blank should have favicon on the tab btw, I think it was changed after this was landed because it caused a lot of UI jank witih loading icon for delay loaded tabs.

[cezaraugusto]

@luixxiul test plan for #5431 is described on #5918, but it's still a WIP. It supersed all tab's icon related issues but still needs some tweaks to be mergeable. I'll follow up your request about #5431 and update it accordingly once PR is ready.

@bbondy currently about:blank has the default paper icon (but newtab has not). If your take is to have a real favicon (like Brave logo) lmk and I'll take the work.