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/runtime/jit/jit.cc b/runtime/jit/jit.cc
index b711181..fa5c41d 100644
--- a/runtime/jit/jit.cc
+++ b/runtime/jit/jit.cc
@@ -243,7 +243,7 @@
       classes_.push_back(klass);
       return true;
     }
-    mutable std::vector<mirror::Class*> classes_;
+    std::vector<mirror::Class*> classes_;
   };
 
   if (generate_debug_info_) {