Don't preload hero images

[schlessera]

From @sebastianbenz on ampproject/amp-toolbox#1132:

We can't tell whether a hero image is responsive and is loaded depending
on viewport size. The current implementation will always preload hero
images, which means images might be preloaded even if they are not
visible on the page. This PR reverts this behavior and disables
preload generation for hero images.

The only exception is when an image defines a media attribute.