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The blacklist was introduced by crrev.com/2592233002 to make
thread checks stricter. The blacklist was to throttle the number
of build bot failures.
There are only few providers left and it causes a need for having
|is_enabled_| in MDM. So, remove this check.

BUG=643438

Review-Url: https://codereview.chromium.org/2844373002
Cr-Commit-Position: refs/heads/master@{#468118}