languages/markdown: remove mdx from filetypes

`:checkhealth` output:

```
- ⚠️ WARNING Unknown filetype 'mdx' (Hint: filename extension != filetype).
```

docs/manual/release-notes/rl-0.9.md

@@ -481,6 +481,7 @@ https://github.com/gorbit99/codewindow.nvim
 - Remove invalid filetypes from glsl lsp config (`vert`, `tesc`, `tese`, `frag`,
   `geom`, `comp`).
 - Remove invalid filetype from go lsp config (`gotmpl`).
+- Remove invalid filetype from markdown lsp config (`mdx`).
 
 [itscrystalline](https://github.com/itscrystalline):
 

modules/plugins/languages/markdown.nix

@@ -169,7 +169,7 @@ in {
       vim.lsp = {
         presets = genAttrs cfg.lsp.servers (_: {enable = true;});
         servers = genAttrs cfg.lsp.servers (_: {
-          filetypes = ["markdown" "mdx"];
+          filetypes = ["markdown"];
         });
       };
     })