change!: do not attempt to download remote images (#161)

Removes a workaround for remote images served from URIs without standard
image extensions (e.g.,
https://img.shields.io/github/actions/workflow/status/rust-lang/mdBook/main.yml
hosts an SVG image despite its YAML extension) since Pandoc implemented
a fix in jgm/pandoc#10557 (released in 3.6.3).
This allows removing the dependency on an HTTP client.