help-app: Delete kHelpAppDiscoverTabNotificationAllChannels flag

This feature flag is no longer used or needed.

Bug: b:325158443
Change-Id: I89f1bbbc5ce26af7ade1d2b4b4819f3085ff2c4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5303316


Commit-Queue: Zufeng Wang <zufeng@google.com>
Reviewed-by: Callistus Tan <callistus@google.com>
Cr-Commit-Position: refs/heads/main@{#1263714}