Remove feature that checks whether tree-sitter node "has changes"

Remove feature that checks whether tree-sitter node "has changes"

First of all, we signal error on using an outdated node, so if a node
has changes, calling treesit-node-check would only raise an error.
Besides, in order to properly answer whether a node has changed, we
would have to update the node as the buffer is edited, which we don't
do right now.

* doc/lispref/parsing.texi (Accessing Node Information): Remove
relevant manual text.
* src/treesit.c (Ftreesit_node_check): Remove docstring mentions,
remove the branch for "has-changes".
(syms_of_treesit): Remove has-changes.