🖍 [Story performance] Remove standalone classes

[mszylkowski]

Replaces i-amphtml-story-standalone selectors with :root.

Closes #36049

[amp-owners-bot]

Hey @gmajoulet, @newmuis! These files were changed:

extensions/amp-story/1.0/amp-story.css
extensions/amp-story/1.0/amp-story.js
extensions/amp-story/1.0/test/test-amp-story.js

[newmuis]

FYI the changes in this PR will drop specificity, which could cause breaking changes

[mszylkowski]

Good point. The only thing that changes specificity for stories are the not !important styles which just seem to be the font-size for now, but font-sizes seem to be defined in 3 different places. I'm cleaning that up to just have that in one place (in this PR) but I could keep the specificity at the current value if we think it's useful.

The other properties changing specificity is not breaking anything, since they are all !important and keep the specificity in the right order