Enable coverage_report_view feature flag by default

Ref: #211410 (closed)

What does this MR do?

This MR removes our coverage_report_view feature flag.

Why are we doing that?

After working on some ~performance improvement for the last 2 milestones. We are now confident with the scalability of our code coverage feature.

Users are now able to use the feature to display code coverage on MR diffs. Here how it looks like:

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