🐛amp-consent: Fixed the flash of the leftover CMP Iframe Container (Flash of teal)

[torch2424]

relates to #19969

This essentially reverts #19830 . I kept this PR in a note in case I noticed something weird down the line and I am glad I did 😂I think this should be undone for the following reasons:

1. This was meant to fix amp-iframe can create multiple iframe when scheduled layout more than once #19599 , but we eventually found the actual fix was 🐛Amp-iframe: Only create one iframe #19603

2. updateFixedLayer is only used in amp-live-list. And I think it's more for dynamically incoming multiple fixed elements.

3. updateFixedLayer throws a console error about the element already having a top applied before being removed.

4. removeFromFixedLayer appears to be the better option still, as we are hiding / removing the element, thus it should also be removed from the fixed layer, and not updated since it no longer exists.

The root cause however, is due to how the fixed layer will handle the top styling for elements. When we add it, the top is set to 0px for us. But when we use update to remove (which also removes all the classes), this top is re-applied which causes the flash, because the 100vh element gets pulled to the top, before being removed. removedFromFixedLayer, removes this top styling entirely (which is what we want).

@zhouyx Let me know what you think! 😄

Gifs

Bug

The low gif framerate may not show the bug, but still shows the console error.

Fix

[zhouyx]

Thank you for investigating! Really interesting found. Could you please add a comment to explain the reason and so we don't change it again. Basically say don't touch this : )