revert: "feat(treesitter): add foldtext with treesitter highlighting"

This reverts commit 9ce1623 in favor of #20750.

runtime/doc/news.txt

@@ -123,7 +123,8 @@ BREAKING CHANGES IN HEAD                                    *news-breaking-dev*
 The following breaking changes were made during the development cycle to
 unreleased features on Nvim HEAD.
 
-• ...
+• Removed `vim.treesitter.foldtext` as transparent foldtext is now supported
+  https://github.com/neovim/neovim/pull/20750
 • ...
 
 ==============================================================================
@@ -223,8 +224,6 @@ The following new APIs and features were added.
   • |vim.treesitter.query.edit()| allows live editing of treesitter
     queries.
   • Improved error messages for query parsing.
-  • |vim.treesitter.foldtext()| applies treesitter highlighting to
-    foldtext.
 
 • |vim.ui.open()| opens URIs using the system default handler (macOS `open`,
   Windows `explorer`, Linux `xdg-open`, etc.)

runtime/doc/treesitter.txt

@@ -652,16 +652,6 @@ foldexpr({lnum})                                   *vim.treesitter.foldexpr()*
     Return: ~
         (`string`)
 
-foldtext()                                         *vim.treesitter.foldtext()*
-    Returns the highlighted content of the first line of the fold or falls
-    back to |foldtext()| if no treesitter parser is found. Can be set directly
-    to 'foldtext': >lua
-        vim.wo.foldtext = 'v:lua.vim.treesitter.foldtext()'
-<
-
-    Return: ~
-        (`{ [1]: string, [2]: string[] }[]|string`)
-
                                      *vim.treesitter.get_captures_at_cursor()*
 get_captures_at_cursor({winnr})
     Returns a list of highlight capture names under the cursor