Fix deprecation: Passing conditions to delete_all is deprecated

What does this MR do?

Fixes rails 5 deprecation: DEPRECATION WARNING: Passing conditions to delete_all is deprecated and will be removed in Rails 5.1. To achieve the same use where(conditions).delete_all. (called from block (4 levels) in <top (required)> at /builds/gitlab-org/gitlab-ce/spec/models/merge_request_diff_spec.rb:108)

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer