markdown: try to improve markdown folding

- Makes rid of treesitter for markdown folding and use the build-in
  vim-markdown plugin features.
- Removes configuration (and uninstall) marksman.

But, still, as the YAML frontmatter isn't well recognized, the folding
method folds it's last line.

Co-Authored-by: iGor milhit <igor@milhit.ch>

after/ftplugin/markdown.lua

@@ -2,6 +2,7 @@
 --   Use treesitter for folding
 --   Disable the color column
 
-vim.opt.foldmethod="expr"
-vim.opt.foldexpr="nvim_treesitter#foldexpr()"
-vim.opt.foldlevel=0
+-- vim.opt.foldmethod="expr"
+-- vim.opt.foldexpr="nvim_treesitter#foldexpr()"
+vim.g.markdown_folding=1
+-- vim.opt.foldlevel=1

init.lua

@@ -487,12 +487,12 @@ local servers = {
   -- rust_analyzer = {},
   -- tsserver = {},
 
-  marksman = {
-    ft = {
-      'markdown',
-      'pandoc'
-    }
-  },
+  -- marksman = {
+  --   ft = {
+  --     'markdown',
+  --     'pandoc'
+  --   }
+  -- },
 
   lua_ls = {
     Lua = {