Remove DexCode canonicalization map.

DexCode cannot be canonicalized and therefore this map has been
unused for a while. Apparently we forgot to remove it.

Change-Id: Ibe564ebc24acc783201d673d7fca9081f91eee8f
diff --git a/src/main/java/com/android/tools/r8/graph/DexApplication.java b/src/main/java/com/android/tools/r8/graph/DexApplication.java
index eea0169..ca702fe 100644
--- a/src/main/java/com/android/tools/r8/graph/DexApplication.java
+++ b/src/main/java/com/android/tools/r8/graph/DexApplication.java
@@ -306,8 +306,6 @@
     private ClasspathClassCollection classpathClasses;
     private LibraryClassCollection libraryClasses;
 
-    public final Hashtable<DexCode, DexCode> codeItems = new Hashtable<>();
-
     public final DexItemFactory dexItemFactory;
     public ClassNameMapper proguardMap;
     private final Timing timing;