itree.c (check_tree): Simplify

itree.c (check_tree): Simplify

* src/itree.c (struct check_subtree_result): Remove `complete`.
(check_subtree): Remove `max_depth` arg (and adjust callers).
Use 0 as black-depth of empty tree.
Remove redundant `node->parent` check (already performed by the caller).
(check_tree): Replace with `check_tree_common` (update all callers).
Check the root's `parent` field.
(check_tree_no_rb): Delete function, inlined in its sole caller.
(interval_tree_remove): Add call to `check_tree` (without RB checks)
before `interval_tree_remove_fix`.  Move update of `size`
field accordingly.