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_mir_transform/src/coverage/graph.rs

@@ -282,8 +282,8 @@ impl graph::WithPredecessors for CoverageGraph {
 
 rustc_index::newtype_index! {
     /// A node in the control-flow graph of CoverageGraph.
+    #[debug_format = "bcb{}"]
     pub(super) struct BasicCoverageBlock {
-        DEBUG_FORMAT = "bcb{}",
         const START_BCB = 0,
     }
 }