Remove noscript from picture element markup

Problem/Motivation

We are working to add an empty img element inside the picture element in issue #2220865: Add empty img element inside picture element to match up with the current version of the picture specification, and to ensure that the img displayed by the picture element (once supported by browsers) has a proper alt attribute.

Once that change is made, we will have another issue, which is detailed in an issue related to the refactoring of the polyfill for the picture element, Picturefill: https://github.com/ResponsiveImagesCG/picture-element/issues/131

The problem is that for those who are using a browser that supports picture, but have JavaScript disabled, they will get two copies of the image downloaded and displayed, which is really bad.

It's important to note that research (https://gds.blog.gov.uk/2013/10/21/how-many-people-are-missing-out-on-ja...) shows that noscript only works when JS is disabled in the browser, which is about 0.2% of traffic. If JS is enabled but not working, noscript doesn't do anything: that's a much larger percentage, about 0.9% of traffic.

So right now noscript provides a benefit to a small number of people: the 0.2% of traffic where JS is disabled in the browser. As browsers start to implement picture support, though, noscript will become a real problem for those people, as it will cause them to see two copies of an image. A better solution is to drop noscript from our markup. That means that 0.2% of traffic will see alt text instead of an image, but that's probably better than getting two copies of an images downloaded and displayed, which slows down page rendering time and screws up the layout.

Proposed resolution

Remove noscript from our markup for the picture element.

Remaining tasks

- Modify markup to remove noscript element
- Remove supporting functions that generate the fallback image in the noscript element
- Modify tests to no longer check for the noscript element or fallback image.

User interface changes

Initially, the 0.2% of site traffic that actually has JavaScript disabled will see alt text rather than an image. However once picture is supported, that is a better option than the alternative, which is that those who have JavaScript disabled in browser that supports picture would see two copies of the same image.

API changes

Once #2220865: Add empty img element inside picture element is committed, our markup would look like this:

<picture>
  <source media="some media query" src="some-image.jpg" />
  <source media="some other media query" src="some-other-image.jpg" />
  <noscript><img src="fallback-image-for-no-js.jpg" alt="Awesome image" title="Awesome image" />
  <img alt="Awesome image" title="Awesome image" />
</picture>

This issue would change that markup to:

<picture>
  <source media="some media query" src="some-image.jpg" />
  <source media="some other media query" src="some-other-image.jpg" />
  <img alt="Awesome image" title="Awesome image" />
</picture>