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

src/idl_gen_kotlin.cpp

@@ -966,8 +966,6 @@ class KotlinGenerator : public BaseGenerator {
       auto bbgetter = ByteBufferGetter(field.value.type, "bb");
       auto ucast = CastToUsigned(field);
       auto offset_val = NumToString(field.value.offset);
-      auto offset_prefix =
-          "val o = __offset(" + offset_val + "); return o != 0 ? ";
       auto value_base_type = field.value.type.base_type;
       // Most field accessors need to retrieve and test the field offset
       // first, this is the offset value for that: