rustc_middle: Remove trait DefIdTree

This trait was a way to generalize over both `TyCtxt` and `Resolver`, but now `Resolver` has access to `TyCtxt`, so this trait is no longer necessary.

compiler/rustc_codegen_llvm/src/debuginfo/utils.rs

@@ -5,7 +5,7 @@ use super::CodegenUnitDebugContext;
 
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::layout::{HasParamEnv, LayoutOf};
-use rustc_middle::ty::{self, DefIdTree, Ty};
+use rustc_middle::ty::{self, Ty};
 use trace;
 
 use crate::common::CodegenCx;