API: Fix Sytem hooks delete behavior

## What does this MR do?

This corrects the delete API for system hooks. Returning 200 is not the right way indicating a hooks is not found.

## What are the relevant issue numbers?

Discussed in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6861/diffs#609af00c90e3d5241064d1404e3e018a3235634a_64_62

See merge request !6883