Delete gallery-app-pdf-edit-notification

- The flag is expired and no longer used.

Bug: b:40284414
Test: na
Change-Id: I126003cf8e1c5721a2fdd6c9c299931e81c18ea1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5805366


Commit-Queue: Yuki Awano <yawano@google.com>
Reviewed-by: Connie Xu <conniekxu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1345147}