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

[liuliu-dev]

Update on Approach

After investigating the dual close button implementation, we've decided to document the existing architecture instead of consolidating the close buttons. @francinelucca clarified that the dual close button pattern was intentionally designed when fullscreen narrow capabilities were added to Overlay (PR #6108):

• Overlay close button: Handles the fullscreen overlay use case on narrow screens (general Overlay functionality)
• SelectPanel close button: Handles the modal-specific use case (SelectPanel-specific functionality)

Revised Approach: Add documentation/comments to explain the architecture and make future debugging easier, rather than changing the working implementation.