Remove the thir_{tree,flat} hooks.

They were downgraded from queries in #123995 but they can just be
vanilla functions because they are not called in `rustc_middle`.

compiler/rustc_middle/src/hooks/mod.rs

@@ -99,12 +99,6 @@ declare_hooks! {
     /// Will fetch a DefId from a DefPathHash for a foreign crate.
     hook def_path_hash_to_def_id_extern(hash: DefPathHash, stable_crate_id: StableCrateId) -> DefId;
 
-    /// Create a THIR tree for debugging.
-    hook thir_tree(key: LocalDefId) -> String;
-
-    /// Create a list-like THIR representation for debugging.
-    hook thir_flat(key: LocalDefId) -> String;
-
     /// Returns `true` if we should codegen an instance in the local crate, or returns `false` if we
     /// can just link to the upstream crate and therefore don't need a mono item.
     ///