source element move and removing steps are wrong

[annevk]

What is the issue with the HTML Standard?

They start with something along these lines:

If movedNode's next sibling was an img element

but that would mean you'd actually have to pass the next sibling in. However, as @domfarolino discovered implementations don't actually do this https://github.com/whatwg/html/pull/10657/files#r1894423255:

Furthermore, I am actually skeptical that the spec has been doing the right thing in this case anyways. For example, Chromium doesn't track the "old next sibling" of the removed source, and trigger a relevant mutation on it. It triggers relevant mutations on all image children of the picture (https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/html/html_picture_element.cc;l=26-29;drc=a7096322ab1e5c9e70ae7b773faed492ed8461cb) so maybe implementations are compatible with that, and we can simplify all of this anyways.