Deprecate ogbg-code and update to ogbg-code2

This version provides a major change in ogbg-code.

• ogbg-code has been deprecated due to prediction target (i.e., method name) leakage in input AST.
• ogbg-code2 has been introduced that fixes the issue., where the method name and its recursive definition in AST are replaced with a special token _mask_.

We sincerely thank Charles Sutton (@casutton) for finding the data leakage in our dataset.