Convert vestigial "SET_DIFF_DATA" to "SET_DIFF_METADATA"

What does this MR do?

The only place that SET_DIFF_DATA is used is in the metadata action.

That call of the mutation is explicitly called without any diff_files (they're deleted).

The mutation has a check for diff_files, which can never be true. Since that clause never runs, the diffFiles are never modified.

This removes all of those unrunnable sections of code, and it renames SET_DIFF_DATA to SET_DIFF_METADATA, since that's the only part that still runs.

Screenshots (strongly suggested)

N/A, all deletes/refactoring.

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team