Fix -Wdefaulted-function-deleted warning

../../components/zucchini/disassembler.h:100:3: warning: explicitly defaulted
default constructor is implicitly deleted [-Wdefaulted-function-deleted]
  ReferenceGroup() = default;
  ^
../../components/zucchini/disassembler.h:144:23: note: default constructor of
'ReferenceGroup' is implicitly deleted because field 'traits_' has no default
constructor
  ReferenceTypeTraits traits_;
                      ^

Bug: 890307
Change-Id: Ic7ec04e175f4fb3cc9a32e6a43115485f671b72f
Reviewed-on: https://chromium-review.googlesource.com/1256805Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595813}

components/zucchini/disassembler.h

@@ -97,8 +97,6 @@ class ReferenceGroup {
   using WriterFactory = std::unique_ptr<ReferenceWriter> (Disassembler::*)(
       MutableBufferView image);
 
-  ReferenceGroup() = default;
-
   // RefinedGeneratorFactory and RefinedReceptorFactory don't have to be
   // identical to GeneratorFactory and ReceptorFactory, but they must be
   // convertible. As a result, they can be pointer to member function of a