Cleanup the PG_VERSION_NUM logic in source files

[tuhaihe]

In the source files, there are several instances where the PG_VERSION_NUM is checked, as illustrated below:

#if PG_VERSION_NUM >= 90600
#include "nodes/pathnodes.h"
#endif
#include "nodes/pg_list.h"
#if PG_VERSION_NUM < 90600
#include "nodes/relation.h"
#endif
#include "utils/rel.h"

It may be beneficial to clean up this old code to maintain a cleaner codebase. Just a suggestion for your consideration.

[MisterRaindrop]

ok I will do it