[refactor](olap-scan-node) Refactor olap scannode (#7131)

1. Delete useless variables
2. Add const modifier for read-only function
3. Delete the empty destructor, the compiler will automatically generate it, refer to the 3/5/0 rule: 
    [https://en.cppreference.com/w/cpp/language/rule_of_three]
4. It is recommended to add the override keyword (instead of the virtual keyword) to the subclass virtual function. 
    Override will let the compiler help check and improve security. This is also the reason why C++11 introduces override

be/src/exec/exec_node.cpp

@@ -656,6 +656,10 @@ Status ExecNode::QueryMaintenance(RuntimeState* state, const std::string& msg) {
     return state->check_query_state(msg);
 }
 
+Status ExecNode::get_next(RuntimeState* state, RowBatch* row_batch, bool* eos) {
+    return Status::NotSupported("Not Implemented get batch");
+}
+
 Status ExecNode::get_next(RuntimeState* state, vectorized::Block* block, bool* eos) {
     return Status::NotSupported("Not Implemented get block");
 }