Fix unused variable warnings across code generators

Removing `_LIBCPP_DISABLE_UNUSED_STRUCT_WARNINGS` from libc++ enables
`[[gnu::__warn_unused__]]` on standard library types (such as
`std::string` and `std::set`), surfacing `-Wunused-variable` warnings.
See [upstream LLVM commit](https://github.com/llvm/llvm-project/commit/ab8b037ef7b398aa864959a69f328fa69b567f74) for reference.

- Remove unreferenced variable declarations whose initializers have no
  side effects (`generated_functions`, `offset_prefix`, `offset_size`,
  `filebase`, `field_name`, `offset`, `ts_file_path_rel`,
  `ts_file_path`, and `union_accessor`).
- Annotate variables initialized via helper or member calls with
  `[[maybe_unused]]` (`offset_str`, `get_field`, `qualified`,
  `struct_var`, `struct_type`, `type`, `def_mod_name`, and
  `ts_file_path`).

PiperOrigin-RevId: 999136370

grpc/src/compiler/ts_generator.cc

@@ -504,7 +504,6 @@ grpc::string GenerateInterface(grpc_generator::File* file,
                                const grpc::string& filename) {
   grpc::string output;
 
-  std::set<grpc::string> generated_functions;
   std::map<grpc::string, grpc::string> vars;
 
   vars["PATH"] = file->package();