Disable support for Gitaly PostReceivePack

Conversation: https://gitlab.com/gitlab-org/gitaly/issues/353

What does this MR do?

It disallows the use of Gitaly's PostReceivePack RPC, regardless of any feature flags. Rationale is in https://gitlab.com/gitlab-org/gitaly/issues/172.

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

Why was this MR needed?

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