Remove support for AllowBelowNormalFromBrowser experiment group.
Remove support for AllowBelowNormalFromBrowser experiment group.

The experiment is complete and no noticeable difference was seen between
BELOW_NORMAL and IDLE, but IDLE was selected to be the one that sticks.

Experiment results, analysis, and conclusion @
https://docs.google.com/document/d/1EgwEv0bRSzX9sNnD0wmSodKIQOHXT-umUKKneb6sTFI/edit

BUG=459600

Review URL: https://codereview.chromium.org/1249683002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#339680}

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