Remove the kBrowserSideDownloadPolicySandbox feature

It was meant to be removed in M111, but lingered past then. This CL
removes it as a clean-up.

R=arthursonzogni@chromium.org

Bug: 1357366
Change-Id: I265865f76fef8b8ea24ad75d176f17a611820f8f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4345437


Auto-Submit: Dominic Farolino <dom@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118870}