Libraries: Use default constructors/destructors in LibJS

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#cother-other-default-operation-rules

"The compiler is more likely to get the default semantics right and
you cannot implement these functions better than the compiler."

Userland/Libraries/LibJS/Bytecode/Generator.h

@@ -194,7 +194,7 @@ public:
 
 private:
     Generator();
-    ~Generator();
+    ~Generator() = default;
 
     void grow(size_t);
     void* next_slot();