Cleanup after group level productivity analytics is verified

Productivity Analytics was moved from the instance level to the group level under Issue #196454 (closed) (see MR !25329 (merged)). Some residual technical debt needs to be addressed:

• Clean up old productivity analytics backend code (ee/app/controllers/analytics/productivity_analytics_controller.rb)
• Clean up group_level_productivity_analytics feature flag.
• Remove group dropdown from PA frontend.