[iOS] Disable kSetUpListShortenedDuration

Previously a decision to launch this feature was made and it was enabled
by default on the client. After re-checking metrics over a longer
analysis window, decision has been made to ramp the feature down.

Bug: None
Change-Id: I430c28960e0c0afd56417c789f48f28e5f7902ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6512556
Reviewed-by: Scott Yoder <scottyoder@google.com>
Commit-Queue: Hira Mahmood <hiramahmood@google.com>
Cr-Commit-Position: refs/heads/main@{#1455905}