Enable gitaly_post_upload_pack by default

What does this MR do?

Enable gitaly_post_upload_pack by default.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

The goal is to remove this feature toggle. Before doing that we want to see if anybody experiences problems with it 'in the wild'. Based on gitlab.com behavior we expect no problems.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

gitaly#184 (closed)