Remove contributor additions/deletions from Repository API

What does this MR do?

This removes the contributor additions and deletions fields from the Repository API "contributors" response. These fields have been considered deprecated since %13.4.

After being introduced in 2014, these fields only returned data for a few months before they were set to 0, presumably due to technical complications that we had originally hoped to resolve quickly. Unfortunately it seems those technical complications have only increased; now addition/deletion data must be retrieved from Gitaly for each commit, which would be slow and burdensome. The simplest solution at this point is to remove the 2 fields from the API response.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #233119 (closed)