Make #[debug_format] an attribute in newtype_index

This removes the `custom` format functionality as its only user was
trivially migrated to using a normal format.

If a new use case for a custom formatting impl pops up, you can add it
back.

compiler/rustc_middle/src/thir.rs

@@ -35,8 +35,8 @@ macro_rules! thir_with_elements {
         $(
             newtype_index! {
                 #[derive(HashStable)]
+                #[debug_format = $format]
                 pub struct $id {
-                    DEBUG_FORMAT = $format
                 }
             }
         )*