multipaste: Remove copied-to-clipboard confirmation toast

Based on guidance from the notifiers team, we are not moving forward
with showing a toast whenever a user copies an item to the clipboard.
This CL removes the flag-gated toast that we previously introduced.

Bug: b/261760656
Change-Id: Icd61b9d8dbdada7816c9b9dbc27456d4625d56cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4550012


Commit-Queue: Colin Kincaid <ckincaid@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147404}