Responsive images: when creating `srcset` do not exclude the image size which...

Responsive images: when creating `srcset` do not exclude the image size which is in the `src` attribute even when it is larger than `max_srcset_image_width`.

Props joemcgill.
Fixes #35108 for trunk.
Built from https://develop.svn.wordpress.org/trunk@36110


git-svn-id: https://core.svn.wordpress.org/trunk@36075 1a063a9b-81f0-0310-95a4-ce76da25c4cd