Remove the contextual hack in tree-sitter fontification

Remove the contextual hack in tree-sitter fontification

We now have a better facility that can replace the contextual
hack. The C part is in the previous commit, and the Lisp part work
will be in the next commit.

* doc/lispref/modes.texi (Parser-based Font Lock): Update manual.
* lisp/progmodes/js.el (js--treesit-font-lock-settings)
* lisp/progmodes/python.el (python--treesit-settings)
* lisp/progmodes/ts-mode.el (ts-mode--font-lock-settings): Stop marking
contextual nodes.
* lisp/treesit.el (treesit-font-lock-contextual-post-process): Remove
function.
(treesit-font-lock-fontify-region): Remove code processing contextual
nodes.