🗑 [story-ads] Cleanup auto progress experiment

[calebcordry]

Gets rid of all the branches and puts everything behind a binary isExperimentOn() check. Also hardcodes the chosen time value of 10 seconds.

partial #33969

[amp-owners-bot]

Hey @Jiaming-X! These files were changed:

extensions/amp-ad-network-adsense-impl/0.1/amp-ad-network-adsense-impl.js

Hey @jeffkaufman! These files were changed:

extensions/amp-ad-network-adsense-impl/0.1/amp-ad-network-adsense-impl.js
extensions/amp-ad-network-doubleclick-impl/0.1/amp-ad-network-doubleclick-impl.js

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

extensions/amp-story/1.0/amp-story-page.js
extensions/amp-story/1.0/amp-story-system-layer.css
extensions/amp-story/1.0/progress-bar.js
extensions/amp-story/1.0/test/test-amp-story-page.js
extensions/amp-story/1.0/test/test-amp-story-progress-bar.js

[ampprojectbot]

Warning: disparity between this PR Percy build and its main build

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