Get rid of kLoadingPerformanceScenario feature

The LoadingScenario will be written by default. It's better for features
that read the scenario to use feature flags, so that different readers
can use different flags, and experiments will only measure the effects
of policy changes without including the overhead of calculating the
scenario state.

Bug: 365586676
Change-Id: Ia892011822dbdf5e4338847f01c28e82eeeeddda
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6000400
Reviewed-by: Patrick Monette <pmonette@chromium.org>
Commit-Queue: Joe Mason <joenotcharles@google.com>
Reviewed-by: Jiahe Zhang <jiahe.zhang@intel.com>
Cr-Commit-Position: refs/heads/main@{#1381233}