[Code Health] Clean up stale kEolIncentiveSettings feature flag

Feature flag `kEolIncentiveSettings` has been disabled by default for
over 2 years. Removing the feature and updating the code associated with
it.

Bug: 356234326
Change-Id: Ic588d69c081f859aec2a1dd8777c5e61e27a9821
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6637682
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Andre Le <leandre@chromium.org>
Auto-Submit: Aaron Selya <selya@google.com>
Reviewed-by: Matthew Mourgos <mmourgos@chromium.org>
Reviewed-by: Dana Fried <dfried@chromium.org>
Reviewed-by: Luc Nguyen <lucnguyen@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Aaron Selya <selya@google.com>
Cr-Commit-Position: refs/heads/main@{#1478707}