Remove obsolete TODO(vabr) from tab specific settings

There are a few TODOs in the code related to tab specific settings
left from the time of componentising them. Those TODOs were related to
planned structural changes in the code, which did not happen in the
end and did not block the componentisation.

Those TODOs are unlikely to provide any value in the future, so this
CL removes them.

Bug: 387075, 908813
Change-Id: Iba5f2600d1a025cf4531c5ca0dbfb6b3fd269789
Reviewed-on: https://chromium-review.googlesource.com/c/1352352
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Martin Šrámek <msramek@chromium.org>
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611708}