Remove listing pages background image

Summary

At PLANET-6725 we removed the background image and the skewed overlay on all listing pages and applied a white background image to align with the design mockups. But it's not enabled by default, controlled by a feature flag under Planet 4 > Information architecture. It's time to phase out the feature flag and enable this by default.

Requirements

• Remove the feature flag from the admin panel.
• All listing pages should now have the white background design.
• Remove all obsolete code of this feature flag.