Remove unused/undefined ImmutableCFOptions() (#6086)

Summary:
default constructor not used or even defined
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6086

Differential Revision: D18695669

Pulled By: pdillinger

fbshipit-source-id: 6b6ac46029f4fb6edf1c11ee6ce1d9f172b2eaf2

options/cf_options.h

@@ -20,7 +20,6 @@ namespace rocksdb {
 // of DB. Raw pointers defined in this struct do not have ownership to the data
 // they point to. Options contains std::shared_ptr to these data.
 struct ImmutableCFOptions {
-  ImmutableCFOptions();
   explicit ImmutableCFOptions(const Options& options);
 
   ImmutableCFOptions(const ImmutableDBOptions& db_options,