Remove treesit-disabled-modes and change treesit-should-enable-p

Remove treesit-disabled-modes and change treesit-should-enable-p

Per emacs-devel discussion, remove treesit-disabled-modes and let
major modes to provide tree-sitter switches.  I also decided to add
treesit-max-buffer-size to elisp manual despite it being a user
option.  Though we should still add it to the user manual.

* doc/lispref/parsing.texi (Parsing Program Source): Update manual to
remove entries for treesit-diabled-modes and add
treesit-max-buffer-size.  Also update treesit-should-enable-p.
* lisp/treesit.el (treesit-disabled-modes): Remove user option.
(treesit-maximum-size): Change to treesit-max-buffer-size.
(treesit-should-enable-p): Change to treesit-can-enable-p and remove
checks of treesit-disabled-modes.