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_infer/src/infer/region_constraints/leak_check.rs

@@ -357,14 +357,14 @@ impl<'tcx> SccUniverse<'tcx> {
 }
 
 rustc_index::newtype_index! {
+    #[debug_format = "LeakCheckNode({})"]
     struct LeakCheckNode {
-        DEBUG_FORMAT = "LeakCheckNode({})"
     }
 }
 
 rustc_index::newtype_index! {
+    #[debug_format = "LeakCheckScc({})"]
     struct LeakCheckScc {
-        DEBUG_FORMAT = "LeakCheckScc({})"
     }
 }