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_grpc.cpp

@@ -149,7 +149,7 @@ class FlatBufPrinter : public grpc_generator::Printer {
   }
 
   void Print(const char *s) {
-    if (s == nullptr || std::strlen(s) == 0) { return; }
+    if (s == nullptr || *s == '\0') { return; }
     // Add this string, but for each part separated by \n, add indentation.
     for (;;) {
       // Current indentation.