internal: fix and force-disable block validation ;-(

Originally we tried to maintain the invariant that `{}` always match.
That is, that in the parse tree the pair of corresponding `{}` is always
first and last tokens of some nodes.

We had the code to validate that, but apparently it's been broken for
**years** since we introduced tokens/nodes split. Fixing it now makes
some tests fail.

It's unclear if we want to keep this invariant: there's a strong
motivation for breaking it in the following case:

```
use std::{ // unclosed paren

fn main() {

}

} // don't actually want to pair up this with the one from `use`
```

So let's fix the code, but disable it for the time being

crates/syntax/src/lib.rs

@@ -162,10 +162,6 @@ impl SourceFile {
         let (green, mut errors) = parsing::parse_text(text);
         let root = SyntaxNode::new_root(green.clone());
 
-        if cfg!(debug_assertions) {
-            validation::validate_block_structure(&root);
-        }
-
         errors.extend(validation::validate(&root));
 
         assert_eq!(root.kind(), SyntaxKind::SOURCE_FILE);