Made codegen always output a file, even on an empty schema.

Previously, we had a check to simply skip such files, but this
tends to make build systems unhappy.

This only affects C++ and JS, since other language output per-class
files.

Change-Id: I54224642725bbafb9f6e1654ed3693e62ca9f7d7
Tested: on Linux.

include/flatbuffers/code_generators.h

@@ -95,8 +95,6 @@ class BaseGenerator {
 
   static const char *FlatBuffersGeneratedWarning();
 
-  bool IsEverythingGenerated() const;
-
   static std::string FullNamespace(const char *separator, const Namespace &ns);
 
   static std::string LastNamespacePart(const Namespace &ns);

include/flatbuffers/flatc.h

@@ -42,6 +42,7 @@ class FlatCompiler {
     const char *generator_opt_short;
     const char *generator_opt_long;
     const char *lang_name;
+    bool schema_only;
     GenerateFn generateGRPC;
     flatbuffers::IDLOptions::Language lang;
     const char *generator_help;