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/member_constraints.rs

@@ -55,8 +55,8 @@ pub(crate) struct NllMemberConstraint<'tcx> {
 }
 
 rustc_index::newtype_index! {
+    #[debug_format = "MemberConstraintIndex({})"]
     pub(crate) struct NllMemberConstraintIndex {
-        DEBUG_FORMAT = "MemberConstraintIndex({})"
     }
 }