Summary
As the title says, I see no way to get Markdown content without HTML or Markdown (including frontmatter), I didn't find a custom plugin for that either. I guess you could use excerpt with an enormous value, but that's counterintuitive.
Basic example
returns the whole Markdown structure including frontmatter.
html on the other hand returns HTML version of it without frontmatter.
Motivation
It supports cases such as getting content ready for e.g. indexing for search or RSS in a understandable (not "Why are you getting an excerpt with 999999 size there?") way without the need of postprocessing with JS.