Clean up force_show_system_bar_for_bubble

This CL cleans up the feature flag of force_show_system_bar_for_bubble
since it has been enabled since the target release (25Q3).

Bug: 329124127
Fix: 429949000
Flag: EXEMPT clean up
Test: presubmit
Change-Id: I109cdd412f0d2f93f45dd94b6ed42c507b6fd992