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_borrowck/src/dataflow.rs

@@ -108,8 +108,8 @@ impl_visitable! {
 }
 
 rustc_index::newtype_index! {
+    #[debug_format = "bw{}"]
     pub struct BorrowIndex {
-        DEBUG_FORMAT = "bw{}"
     }
 }