Consolidate SelectPanel close button implementation after fullscreen on narrow feature flag removal

[liuliu-dev]

Summary

After the primer_react_select_panel_fullscreen_on_narrow feature flag is removed, we need to consolidate the dual close button implementation in SelectPanel to improve maintainability and remove CSS workarounds.

Background

Currently, SelectPanel has two close button options:

1. Close button in AnchoredOverlay
2. Close button in SelectPanel wrapper

A temporary CSS alignment fix was added in #6327, but this should be cleaned up once the feature flag is removed.

Tasks

• Remove primer_react_select_panel_fullscreen_on_narrow feature flag
• Consolidate to a single close button implementation
• Remove CSS workaround for dual close button alignment