Remove an UNIMPLEMENTED(WARNING) that isn't worth implementing.

We _could_ fix this, but it would be ugly, potentially expensive, and it's
really not obvious there's any point. (At least until we have to worry about
DexFile*s not owned by the ClassLinker.)

Change-Id: I2404bfe15e36bba0a6d1d362a465f2fccf1223eb
diff --git a/src/dex_file.cc b/src/dex_file.cc
index 7c49644..a84c316 100644
--- a/src/dex_file.cc
+++ b/src/dex_file.cc
@@ -359,9 +359,10 @@
 }
 
 DexFile::~DexFile() {
-  if (dex_object_ != NULL) {
-    UNIMPLEMENTED(WARNING) << "leaked a global reference to an com.android.dex.Dex instance in " << location_;
-  }
+  // We don't call DeleteGlobalRef on dex_object_ because we're only called by DestroyJavaVM, and
+  // that's only called after DetachCurrentThread, which means there's no JNIEnv. We could
+  // re-attach, but cleaning up these global references is not obviously useful. It's not as if
+  // the global reference table is otherwise empty!
 }
 
 jobject DexFile::GetDexObject(JNIEnv* env) const {