win: Remove unused taskbar_icon_finder

This looks to have been added for the Win10 inactive user toast
experiment, but not removed when that was removed.

Bug: 330118244
Change-Id: If452accc12068e3ec819710820a764eb4e76c3b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5407671


Reviewed-by: Jesse McKenna <jessemckenna@google.com>
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280267}