Fix updating string dex cache array for no class table case

The issue was that array classes do not have a dex cache. Also removed
some unnecessary mutables.

Bug: 22858531
Change-Id: I2bc45f019e064b6e562c8f158cc2ac8c0e513afd
diff --git a/compiler/image_writer.cc b/compiler/image_writer.cc
index d63fed5..73574ba 100644
--- a/compiler/image_writer.cc
+++ b/compiler/image_writer.cc
@@ -859,7 +859,7 @@
     return true;
   }
 
-  mutable std::unordered_set<mirror::Class*> classes_to_prune_;
+  std::unordered_set<mirror::Class*> classes_to_prune_;
   ImageWriter* const image_writer_;
 };