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/constraints/mod.rs

@@ -115,13 +115,13 @@ impl<'tcx> fmt::Debug for OutlivesConstraint<'tcx> {
 }
 
 rustc_index::newtype_index! {
+    #[debug_format = "OutlivesConstraintIndex({})"]
     pub struct OutlivesConstraintIndex {
-        DEBUG_FORMAT = "OutlivesConstraintIndex({})"
     }
 }
 
 rustc_index::newtype_index! {
+    #[debug_format = "ConstraintSccIndex({})"]
     pub struct ConstraintSccIndex {
-        DEBUG_FORMAT = "ConstraintSccIndex({})"
     }
 }