Remove unnecessary `explicit` qualifiers on constructors.

Change-Id: Id12e392ad50f66a6e2251a68662b7959315dc567
diff --git a/runtime/mirror/object.cc b/runtime/mirror/object.cc
index 87fb5ba..df680b5 100644
--- a/runtime/mirror/object.cc
+++ b/runtime/mirror/object.cc
@@ -107,7 +107,7 @@
 // An allocation pre-fence visitor that copies the object.
 class CopyObjectVisitor {
  public:
-  explicit CopyObjectVisitor(Thread* self, Handle<Object>* orig, size_t num_bytes)
+  CopyObjectVisitor(Thread* self, Handle<Object>* orig, size_t num_bytes)
       : self_(self), orig_(orig), num_bytes_(num_bytes) {
   }