This flag has been on by default for a while, along with
FLAG_track_prototype_users which provides the prerequisite
infrastructure. We are now sure that this is the direction
we want to go in, so in order to simplify the code, this CL
drops the respective flag-off code paths.

Review-Url: https://codereview.chromium.org/1993913002
Cr-Commit-Position: refs/heads/master@{#36445}