Two AST nodes for member function call is unnecessary

In the `VisitMemberExpr()` function an entry is added for accessing members. However, if this is a method call, then it is already inserted as a function call. So in this case adding it to the database as a member expression is unnecessary:
```.cpp
obj->func(); // Function call and member expression. The latter one is unnecessary.
obj->func; // Only a member expression. It is fine.
```