Remove bogus ImageWriter friend declaration.

There is no art::ImageWriter, only art::linker::ImageWriter.

Test: Rely on TreeHugger.
Change-Id: I59b02debb861b783408ddcbcc90534d9a3b7907e
diff --git a/runtime/mirror/object.h b/runtime/mirror/object.h
index ca8867d8..f727085 100644
--- a/runtime/mirror/object.h
+++ b/runtime/mirror/object.h
@@ -33,7 +33,6 @@
 
 class ArtField;
 class ArtMethod;
-class ImageWriter;
 class LockWord;
 class Monitor;
 struct ObjectOffsets;
@@ -782,7 +781,6 @@
   uint32_t x_xpadding_;    // For 8-byte alignment. TODO: get rid of this.
 #endif
 
-  friend class art::ImageWriter;
   friend class art::Monitor;
   friend struct art::ObjectOffsets;  // for verifying offset information
   friend class CopyObjectVisitor;  // for CopyObject().