Eliminate obsolete `post-stream:refresh` event triggers. These events were marked as TODO for removal after the Glimmer migration and are no longer needed.

This cleanup improves code clarity and reduces unnecessary event emissions, with no impact on functionality since the events were unused by the current post-stream implementation.

Remove usages of `glimmer_post_stream_mode` in tests across various files,
including acceptance and system-level specs, where it was used to toggle
testing behavior. This reduces test complexity and ensures coverage without
dependency on this feature flag, which is being phased out.

Remove deprecated `discourse.post-stream-widget-overrides` and associated
legacy widget customizations.

This cleanup reduces technical debt, ensures future maintainability, and
aligns with the latest framework improvements.