[Permissions] Remove hard coded threshold values.

We store those thresholds in the metadata of the CPSSv1 models. The
fallback branch is not used:
https://uma.googleplex.com/p/chrome/timeline_v2?sid=76ff9efbfe3226dfd749eb44a3eec250

OBSOLETE_HISTOGRAM[Permissions.PredictionService.PredictionThresholdSource]=After this change it is no longer necessary to track usage of hardcoded vs metadata stored thresholds.

Bug: 414792191
Change-Id: I4b74bde203a8633552dffda19b76a798f854a71e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6501654
Reviewed-by: Ravjit Uppal <ravjit@chromium.org>
Commit-Queue: Judith Hemp <hempjudith@google.com>
Cr-Commit-Position: refs/heads/main@{#1457442}