Disabled Tajo-master-CODEGEN-build

[Hyunsik Choi]

Hi folks,

Codegen, which is an experimental feature, is broken for a while. So, I
just disabled this post-commit test [1]. After we fix it, we can enable it
later.

[1] https://builds.apache.org/job/Tajo-master-CODEGEN-build/

Best regards,
Hyunsik