Remove feature flag `data_transfer_monitoring_mock_data`

What does this MR do and why?

Contributes to #397693 (closed)

Problem

The feature flag was used to support frontend with a temporary data for testing. The work is completed, the feature flag is not needed anymore.

Solution

Remove the feature flag.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.