[Bitcode] Remove unused declaration writeGlobalVariableMetadataAttachment (NFC)

The declaration was introduced without a corresponding definition on
May 31, 2016 in commit cceae7feda.

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -373,7 +373,6 @@ private:
   void writeModuleMetadata();
   void writeFunctionMetadata(const Function &F);
   void writeFunctionMetadataAttachment(const Function &F);
-  void writeGlobalVariableMetadataAttachment(const GlobalVariable &GV);
   void pushGlobalMetadataAttachment(SmallVectorImpl<uint64_t> &Record,
                                     const GlobalObject &GO);
   void writeModuleMetadataKinds();