This removes the fallbacks in `wp_get_attachment_image()` and in
`wp.media.string.props` which attempt to generate an `alt` value
from the image caption or title if an `alt` attribute isn't explicitly
set.
This allows for image HTML to be generated that contains an empty `alt`
value, i.e., `alt=""` which is much preferable for screen readers than
reading redundant content in the case of a caption, or when reading the
image title, which is often generated from the filename and not helpful
as `alt` text.
Props odie2, joedolson, rianrietveld, afercia, iamjolly, joemcgill.
Fixes#34635.
git-svn-id: https://develop.svn.wordpress.org/trunk@38812 602fd350-edb4-49c9-b593-d223f7449a82
788bb093f2