Stop measuring the timing for reset approval service

What does this MR do and why?

This MR removes the performance instrumentation that was added to measure the duration of operations in the MergeRequests::ResetApprovalsService.

The timing measurements were added behind the log_merge_request_reset_approvals_duration feature flag to gather performance data. Now that the data collection is complete, this MR cleans up:

• The feature flag definition (log_merge_request_reset_approvals_duration.yml)
• All measure_duration and measure_duration_accumulate wrapper calls throughout the service
• The logging infrastructure (log_hash_metadata_on_done, duration_statistics, etc.)
• Associated test coverage for the duration logging functionality

This simplifies the code by removing the instrumentation overhead and returning the service to its core functionality.

Related Issues

• Feature issue: https://gitlab.com/gitlab-org/gitlab/-/issues/548046
• Rollout issue: #579591
• Introduced in: !210830 (merged)

Changelog: other