Remove position-area-computed-insets.html from anchor positioning focus area

[emilio]

Test List

https://wpt.fyi/results/css/css-anchor-position/position-area-computed-insets.html

Rationale

It depends on TypedOM, which we don't just yet (we're on it).

We implement the right thing, though it's not testable via getComputedStyle because of the resolved value behavior of those properties.

[nt1m]

Seems fine to me

[nairnandu]

Reviewed with @bfgeek. lgtm for Chromium as well