Remove test summary widget usage ping feature flag

What does this MR do and why?

For #326058 (closed)

This MR removes the usage_data_i_testing_summary_widget_total feature flag because feature flags are no longer necessary for every usage ping event.

This feature flag controlled the tracking of the i_testing_summary_widget_total usage ping event, which counts expansions of the test summary MR widget.

Screenshots or screen recordings

There should be no visual changes, but here's the widget for reference:

Screen_Recording_2021-10-05_at_16.01.40

How to set up and validate locally

1. git checkout 326058-remove-test-summary-widget-expansions-usage-data-feature-flag
2. set up unit test reports for a project OR clone this test project
3. create a merge request for a branch that changes unit test results (create a merge request for feature-branch in the test project)
4. ensure that pipelines ran for the base commit and head commit of the merge request's branch
5. navigate to the merge request and locate the test summary MR widget
6. open your browser's devtools to the network tab to view the list of HTTP requests made by the current page
7. click the Expand button on the test summary MR widget
8. a new request to /api/v4/usage_data/increment_unique_users should appear in the list of HTTP requests, this shows that the expansion event is still being tracked even though the feature flag has been removed

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.