Remove unused variables (#5382)

- Fix GenerateTextFromTable (aliasing typo)
- Fix unused variable in idl_gen_dart.cpp
- Fix std::string passing (should be non-const value or const-reference)
- Remove unused variables

src/idl_gen_js_ts.cpp

@@ -214,7 +214,7 @@ class JsTsGenerator : public BaseGenerator {
     std::string &code = *code_ptr;
     std::string &exports = *exports_ptr;
     for (auto it = sorted_namespaces.begin(); it != sorted_namespaces.end();
-         it++) {
+         ++it) {
       if (lang_.language == IDLOptions::kTs) {
         if (it->find('.') == std::string::npos) {
           code += "import { flatbuffers } from \"./flatbuffers\"\n";