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This mode is always enabled now, so we can remove the code for handling
when it is disabled.

BUG=236848
TEST=Tests still pass
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2617893002
Cr-Commit-Position: refs/heads/master@{#442012}