Remove DropNodeKey::kind.

It's not needed, because `next` and `local` fields uniquely identify the
drop. This is a ~2% speed win on the very large program in #134404, and
it's also a tiny bit simpler.

Author: nnethercote

compiler/rustc_mir_build/src/builder/scope.rs

@@ -230,7 +230,6 @@ struct DropNode {
 structDropNodeKey{
 next:DropIdx,
 local:Local,
-kind:DropKind,
 }
 
 implScope{
@@ -291,7 +290,7 @@ impl DropTree {
 let drops = &mutself.drops;
 *self
 .existing_drops_map
-.entry(DropNodeKey{ next,local: data.local,kind: data.kind})
+.entry(DropNodeKey{ next,local: data.local})
 // Create a new node, and also add its index to the map.
 .or_insert_with(|| drops.push(DropNode{ data, next }))
 }