Include PreloadHeroImage among transformers omitted for SSR

[westonruter]

Summary

When SSR is disabled via add_filter( 'amp_enable_ssr', '__return_false' ); I found that the data-hero attributes were still getting added, along with prerendering and the i-amphtml-ssr attributes. This transformer should not be running when SSR is disabled.

The result is also an invalid AMP page since it is not marked as transformed:

Amends #5350 for #5055.

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

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[pierlon]

QA Passed

Disabling SSR does not result in data-hero nor i-amphtml-ssr attributes being added to any image on the page, and the page is valid AMP.

I did notice however that when SSR is disabled, the page becomes invalid AMP when there are amp-bindings:

This seems to be because of the OptimizeAmpBind transformer still running even though SSR is disabled. I'll open a PR to fix this issue.