Remove web performance widget usage data flag

What does this MR do?

For #332402 (closed)

This MR removes the usage_data_i_testing_web_performance_widget_total feature flag, which was introduced to control tracking of the i_testing_web_performance_widget_total usage ping event back when a feature flag was required for every event.

Screenshots or Screencasts (strongly suggested)

No visual changes are expected, but here is the widget anyway:

How to setup and validate locally (strongly suggested)

1. set up browser performance testing on a project - here's a test project
2. open a merge request that changes performance
3. ensure that pipelines ran for the base and head commits of the merge request
4. navigate to the merge request
5. expand the widget
6. verify that the event tracking request was made: a POST to /api/v4/usage_data/increment_unique_users, resulting in a 200 response

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)