WATCHLISTS: Remove PA watcher

Since (Google-internal) chrome-memory-tok was added back in 2021 [1], I
don't think we've actually seen a stream of CLs in PA's directories.
Whatever is responsible for ferrying Gerrit activity seems not to have
write permission for internal groups.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/3129486

Change-Id: I3ce909d1e8c76c79711022c2d624f5d2b6bc3bf5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5601372


Reviewed-by: Mikihito Matsuura <mikt@google.com>
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312728}