FileGenerator::GenerateHeader(): Set `min_header_version` unconditionally

Previously, we were conditionally trying to set `min_header_version` to
the lowest possible value, and relying on a "legacy" Google interface to
determine the file descriptor's syntax version as part of that
determination.

Instead, simply bump the minimum version to 1003000 (1.3.0). This
release was almost 7 years ago. In practice protobuf-c users should not
be shipping pre-compiled .pb-c.c/.pb-c.h files, anyway.

protoc-c/c_file.cc

@@ -117,14 +117,7 @@ FileGenerator::~FileGenerator() {}
 void FileGenerator::GenerateHeader(io::Printer* printer) {
   std::string filename_identifier = FilenameIdentifier(file_->name());
 
-  int min_header_version = 1000000;
-#if GOOGLE_PROTOBUF_VERSION >= 4023000
-  if (FileDescriptorLegacy(file_).syntax() == FileDescriptorLegacy::SYNTAX_PROTO3) {
-#else
-  if (file_->syntax() == FileDescriptor::SYNTAX_PROTO3) {
-#endif
-    min_header_version = 1003000;
-  }
+  const int min_header_version = 1003000;
 
   // Generate top of header.
   printer->Print(