[coverage_report_view] Remove inline code coverage visualization feature flag that is currently enabled by default

What

Remove the :coverage_report_view feature flag. It has already been enabled by default and the rollout steps previously completed in #211410 (closed).

Owners

• Team: Verify:Testing
• Most appropriate slack channel to reach out to: #g_testing
• Best individual to reach out to: @morefice @rickywiens

Expectations

The feature flag will be removed, cleaning up the code.

Roll Out Steps

• Remove feature flag and add changelog entry
• After the flag removal is deployed, clean up the feature flag by running chatops command in #production channel