Remove FledgeNegativeTargeting feature flag.

And leave the feature permanently enabled. It's been enabled by default
on stable for 8 months now, so at this point we don't need to keep the
ability to disable it.

Bug: 40275797
Change-Id: I8543b15dea3a14525adaa147bf85f91cebce06c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5769921
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Commit-Queue: mmenke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1340974}