Remove unnecessary `explicit` qualifiers on constructors.

Change-Id: Id12e392ad50f66a6e2251a68662b7959315dc567
diff --git a/compiler/utils/x86/assembler_x86.h b/compiler/utils/x86/assembler_x86.h
index d9c1b40..37c69fe 100644
--- a/compiler/utils/x86/assembler_x86.h
+++ b/compiler/utils/x86/assembler_x86.h
@@ -205,7 +205,7 @@
 
 class X86Assembler FINAL : public Assembler {
  public:
-  explicit X86Assembler() {}
+  X86Assembler() {}
   virtual ~X86Assembler() {}
 
   /*