Remove item tree tests

They were useful during initial development of the item tree, but
now just cause churn

Author: jonas-schievink

crates/hir_def/src/item_tree.rs

@@ -1,8 +1,6 @@
 //! A simplified AST that only contains items.
 
 mod lower;
-#[cfg(test)]
-mod tests;
 
 use std::{
     any::type_name,