dont set ft=agda for literate agda

[4e554c4c]

Literate agda stored in markdown files (foo.lagda.md) is markdown first, and then agda in small areas. The markdown treesitter parser is designed to work with this when ft=markdown.

Currently when I open a literate agda file, I can change the filetype to markdown and cornelis works fine with the markdown/latex treesitter parsers running concurrently, so I assume only a few changes will be needed to make cornelis work with ft=markdown on startup.