LibJS: Remove unused ExecutionContext::arguments_object

Userland/Libraries/LibJS/Runtime/ExecutionContext.h

@@ -30,7 +30,6 @@ struct ExecutionContext {
     FlyString function_name;
     Value this_value;
     MarkedValueList arguments;
-    Object* arguments_object { nullptr };
     bool is_strict_mode { false };
 };
 

Userland/Libraries/LibJS/Runtime/VM.cpp

@@ -103,7 +103,6 @@ void VM::gather_roots(HashTable<Cell*>& roots)
         for (auto& execution_context : stack) {
             if (execution_context->this_value.is_cell())
                 roots.set(&execution_context->this_value.as_cell());
-            roots.set(execution_context->arguments_object);
             for (auto& argument : execution_context->arguments) {
                 if (argument.is_cell())
                     roots.set(&argument.as_cell());