Remove deltas_only from DiffCollection

Conversation: gitaly#222 (closed)

What does this MR do?

Follow-up to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11201 . This change removes some code we no longer need.

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

Why was this MR needed?

Because removing code is good.

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?

Closes gitaly#224 (closed).